Here I list just a few reasons to think sodomy a chemical addiction. For more reasons, references, and justifications, go here to see the anti-sodomy chapter of my book, Exact Morality for Today. Alternatively, go to Download page for Exact Morality for Today to obtain my whole book for free.
I'd say that the average person is rather disgusted by sodomy, by which I mean behavior that introduces semen into the digestive system (viz. oral sodomy and rectal sodomy). Male homosexuals want to make their behavior seem primarily a gender-preference issue as opposed to a question of whether people should have the right to sodomize. Sodomy is not a pretty picture to most people. Isn't the simplest explanation for why so many people are disgusted by sodomy that there is an evolutionary advantage in viewing the behavior as disgusting, because sodomy is evil?
In the nineteenth-century, standard opinion was that sodomy was an immoral addiction. Thus, the anti-sodomy laws that existed in all the states. Here is the opinion of Charles MacKay, in his Extraordinary Popular Delusions and the Madness of Crowds, on how sodomy helped James I to be screwed-up:
"Robert Kerr, a Scottish youth, was early taken notice of by James I., and loaded with honours, for no other reason that the world could ever discover than the beauty of his person. James, even in his own day, was suspected of being addicted to the most abominable of all offences; and the more we examine the history now, the stronger the suspicion becomes. However that may be, the handsome Kerr, lending his smooth cheek even in public to the disgusting kisses of his royal master, rose rapidly in favour. In the year 1613, he was made Lord High Treasurer of Scotland, and created an English peer by the style and title of Viscount Rochester. Still further honours were in store of him."
True, people as a whole can be mistaken about the truth (the main thesis of Charles MacKay's book), but still, that historically people have mostly viewed sodomy as evil suggests that at the least it is worth investigating scientifically whether in fact it be evil. It would be presumptuous to assume from a misguided assumption that our "scientific" generation can't have an extraordinary popular delusion that whenever our generation differs in view from past generations, it is always the past generations that had the extraordinary popular delusion.
A few not at all ad hominem reasons it seems reasonable to me to view sodomy as a likely enslaving addiction:
1. Semen contains chemicals that could well be addictive. For instance, human seminal vesicles produce more E- and F- type prostaglandins than the rest of the body combined. Many of these are known to be important neuromodulators, affecting mood. Moreover, there has been some evidence that increased PGE1 levels in particular are associated with the narcosis of alcohol addiction. (A study was done showing that drunk mice who were given PGE inhibitors were able to right themselves quicker than other drunk nice, and it is also well known that alcohol temporarily increases PGE levels in the brain.) Apparently, accordingly to Dr. Lani Burkman, semen also contains anandamide, the neurotransmitter whose receptors marijuana affects to produce its high. (Chocolate also contains anandamide.) It is true that the chemistry of addiction is very poorly understood (in fact, alcohol addiction is not even understood well), but given that sodomy happens, that semen contains chemicals which have (poorly understood) effects on the brain suggestive of addiction, and that males have quite an evolutionary advantage in addicting females into wanting to have sex with them, it would I think to anyone stepping back and looking dispassionately at the situation in fact be surprising if sodomy were not chemically addictive.
2. Semen contains chemicals that could increase the terror with which concomitant physical abuse is viewed, thereby increasing the power of the abuser to gain domination over his victim. Indeed, aspirin and similar analgesics work to block sensitivity to pain by blocking prostaglandin E2, which exists copiously in semen. Since men are often physically abusive in relationships, it is reasonable to suppose that semen contains these pain-sensitizing algesics because these abusive men have an evolutionary advantage to their abuse being especially terrifyingly controlling. Algesics such as PGE2 in semen supports the notion that sodomy is about enslavement.
3. The recognized female erogenous zones, namely the clitoris and the G-spot, are located anteriorly in the female sexual apparatus. Presumably, males who hate sodomy do from fear of putting a square peg into a round hole, so to speak, tend to be anterior when having sex, trying to make sex as far away from sodomy as possible. If sodomy is addictive, it is clear that females would evolve to be sexually aroused by sex that suggests it was desired by her own nature as opposed to just adventitiously as a result of having been sodomized, so sodomy being addictive furnishes an explanation for the locations of the female erogenous zones.
4 A distinguishing characteristic of mammals (with a few exceptions) is that they have separate openings for the terminus of the digestive system and the reproductive system. An obvious explanation for why this characteristic (otherwise rather disadvantageous as requiring the products of the kidneys to be dissolved in water, which is not always easy to come by) evolved is that in cloacal animals (such as reptiles) sex itself is substantially addictive much as sodomy is addictive in mammals, and clearly sexual selection is harmed by sex being addictive. This would help explain why, on the whole, mammals have done as well as they have.
5. Paleontology suggests that in human evolution, upright stance evolved prior to increased brain size. This was surprising to scientists, who thought upright stance became useful only after man became smart enough to make and use tools, thereby requiring him to have freer use of the hands. A simple explanation given by the addictiveness of sodomy is that hominid females are more protected from sodomy if they have sex from the front as opposed to the back. Protection from sodomy leads to meaningful sexual selection, which leads to increased selection for intellectual qualities.
6. Biology suggests that sodomy would be more addictive than sex. Histology tells us that the rectum, unlike the vagina, is lined by simple epithelial tissue, the sort of tissue commonly associated with absorbing chemicals well. Also, much of the venous system from the rectum (unlike the venous system from most of the rest of the digestive system) bypasses the liver, allowing chemicals to go directly into the main bloodstream (and thence to the brain) without having to suffer what pharmacologists call the first-pass effect, i.e., the tendency of drugs to be decomposed by the liver into inactive byproducts before they can reach the target organ (that is mainly why some drugs are given rectally as opposed to orally).
7. It is known that prostaglandins are decomposed by the lungs (and to a lesser degree, by the liver). This would explain the tendency of sex to suggest heavy breathing in the popular imagination. It would also explain why deep breathing has religious significance to the Hindus. Moreover, it is well known that a popular practice in sexual deviant circles is to try to increase sexual desire by restricting breathing during sex. In fact, there was an infamous mass murderer in the Midwest (Herb Baumeister) who on the face of it killed not because he wanted to kill his victims per se but because he so much liked to hold his male sodomy partners underwater while he was sodomizing them that large numbers of his partners drowned to death. And it is well known that murderous child molesters and your other sexual-deviant-type killers disproportionately kill by strangulation (I heard this from an FBI or ex-FBI agent on one of those crime shows, probably Investigative Reports on A&E).
8. Scientists have never really investigated whether sodomy be addictive (periodically I go to search engines and search these studies, and I never find anything) in the chemical sense that ordinary people tend to use when they use the word addictive in a useful sense (i.e., not as a synonym for pleasant). On the other hand, many scientists have tried to show that desiring sodomy (or male homosexuality, at any rate) is genetic, and they have failed. For instance, the much heralded gene study turned out to be not reproducible by other scientists (Doubt Cast on Gay Gene). And the reports of brain structure being different in gays shows nothing inasmuch as it is known that alcoholics also have brain structure different from that of non-alcoholics. Of course, not that the tendency to sodomize or to not have strong defenses against being addicted to sodomy is not largely genetic even if the tendency to want to be sodomized is addictive. People can have a genetic predisposition to become alcoholics, yet clearly alcoholism is not genetic. No one who never ingested alcohol ever became an alcoholic.
9. On the face of it, it seems strange that in mammals the y-chromosome would have split off to itself, making crossover in most of the chromosome impossible, thereby bringing a grinding halt to the continued evolution of the genetic material on it, and in fact tending to cause its decay. A simple explanation is that there is a genetic advantage to a species in not enabling fast evolution of sexual traits exclusive to males, because there is some evil sexual trait exclusive to males which it is very advantageous for a species to stop the continued evolution of. Sodomy being evil explains why it was advantageous for the y-chromosome (which largely regulates testis development) to have stopped evolving–presumably much of the addictiveness of semen stopped evolving at the same time, at least until genes on the other chromosomes took over.
10. It is surprising that females don't have conscious control over whether a copulation will lead to fertilization. Couldn't such control be easily had? And wouldn't it be a very useful defense against rape? The simple explanation is that pregnancy always being a possibility guards against females experimenting with sex, which would excessively put them at risk of being sodomized. Dogs, I understand, have done one better. If a female dog has sex but does not become pregnant, the dog likely will undergo painful pseudopregnancy. This suggests a female dog has control over whether she can become pregnant (a useful anti-rape defense), but that she still will be discouraged from (imprudently) trying superfluous sex.
11. Male gonads are exposed in most mammals. Presumably, algesic chemicals produced in the gonads have pain-sensitizing effects. Thus, male gonads being vulnerable tends to make sodomizers cowardly, as is useful to the species. It is relevant that (according to the McGraw-Hill Encyclopedia) the placental terrestrial mammals who lack seminal vesicles (so all the addictive prostaglandins presumably have to be produced by the testes) are those which are in the family carnivora. Obviously cowardice is more harmful when you're a carnivore. The reason the panda is so cute is probably that it is apparently the only non-carnivorous terrestrial placental mammal who (because it is in the family carnivora) lacks seminal vesicles.
Update (October 2005): Actually, since writing (11) I have found information indicating that the Aye-aye (cf. Aye-aye at Duke University) and the camelid family (camels, llamas, alpacas, vicunas, guanacos) also lack seminal vesicles. (Searching on the internet revealed no additional seminal-vesicle-lacking placental mammals I had not known about, but perhaps there could be others.) How camelids came to lack seminal vesicles is understandable inasmuch as they are intrauterine inseminators, and the uterus has evolved a very negative response to prostaglandins, that includes uterine cramping, vomiting, and diarrhea (obvious antisodomy defenses). In fact, doctors performing intrauterine insemination must first wash the prostaglandins off the sperm in order to keep their patients from getting very sick. Thus, once intra-uterine insemination evolves, having too many prostaglandins in semen can be disadvantageous to males. Notwithstanding that doubtless intra-uterine insemination when it evolved strongly and usefully selected against addictive semen, Camelids may in the long-run pay for this tendency, though, as over time the chemicals of semen might evolve to be addictively absorbed by the uterus, which not being lined by stratified epithelium, presumably is instrinsically more susceptible to absorbing chemicals than the vagina itself. The more interesting exception is the Aye-aye, an obscure primate native to Madagascar. It is tempting to suppose that the explosion in evolution that occurred shortly after the rise of the primates was triggered by an Aye-aye-like creature that through mutation lost the ability to code for seminal vesicles. Because the creature lived in Madagascar (a small place) the mutation nevertheless by chance was able to succeed notwithstanding males containing the mutation were not as addicting of females. Later, some of these animals crossed into Africa, where being of a species essentially immune to sodomy, they exploded in population size and in the evolution of new useful traits. Then, unfortunately, the mutation reversed itself through another mutation, but not before a very significant evolution of higher tendencies occurred. The particulars of early primate evolution are unclear, though, and seem to revolve about the question of how many times geology allowed early primates to travel to and from Madagascar. See Ancient single origin for Malagasy primates .
Female/female intimacy I think is actually tied up with bisexual feelings. Sperm that goes back-and-forth between females selects for sperm able to survive such travel, and only sperm that has had studly ancestral sperm well-loved by females is likely to have evolved the ability to survive such back-and-forth sex. Thus, female bisexuality tends to select for children with characteristics especially well-loved by females, which are thus likely to be extra desirable. It is a totally different phenomenon from sordid male homosexuality. I am not anti-gay per se, but anti-sodomy. In fact, in my opinion, sodomy harms females more than males inasmuch as females are presumably more commonly sodomized.