Although it ought to be implicit from what follows, I feel that if I don't make an apology at the outset I will be setting myself up for a torrent of unfair accusations. I am not arguing in favor of rape, or pedophilia (though I think our responses to both raise serious criticism, a topic for another discussion). Nor am I arguing for any form of sexual activity (assuming all participants are human) were all involved are not both fully "consensual" but also not fully competent to give consent (by "competent" I mean sober, mentally fit and adult). I should also note that this is not intended as satire--I take these issues as seriously as I take any--
It seems to me that we are just, or nearly at any rate, as bigoted with respect to sexual "perversions" as ever. All that has changed is what was once tolerated is no longer tolerated and vice versa. In particular, modern bigotry is aimed at those who have sex with animals, those who commit incest and those who have multiple sexual partners who wish to enter marriage. If "bestiality" can be proved to be harmful animals then clearly it ought to be objected to for this reason (provided we also stop other actions, such as buying leather and eating meat, that, unlike fucking them or being fucked by them*, are patently harmful to animals). But if we're honest, no one really objects to bestiality for this reason--they do so for the exact same reasons many object to homosexuality.
The case seems the same with polygamy, polyandry and other "unconventional" arraignments. It is true that existing forms polygamy happen to be the product of profoundly patriarchal cultures such as the LDS Church, but I do no think that polygamy is necessarily patriarchal, assuming all involved do so fully voluntarily. Besides, monogamous marriage (as we understand it) is itself a deeply patriarchal institution.
As of the other two, a shallow scientific/rational rationale is provided in argument against incest**--that it increases the chances of birth defects where a child is conceived. In this case I agree that it is irresponsible for two who are closely related to beget offspring (or risk doing so accidently)***; however, no valid argument comes to mind against homosexual incest.
In this respect as in many others I think we should careful not to be too quick to pat ourselves on the back about our supposedly "enlightened values"; not only is the smugness of it unattractive, worse, it prevents us from tackling very real problems left to be dealt with.
*In a documentary--unfortunately I have forgotten the title--that aired on Channel 4 (UK) several women variously describe being humped and given head by various male animals, including dogs and horses. In these cases it is clear the animal in question was a more-than- willing participant.
**Prejudice against incest runs deep in the academic community, where is cloaked in fancy psychoanalytic jargon such as "oedipal complex". Mutations aside, it may turn out there are valid psychological reasons for the avoidance of incest. My suspicion is that this is the case. However, psychology is too poorly developed at this stage to honestly assert this claim with any confidence and even if in the future it becomes able to this fact would hardly justify kind of abuse those who commit incest are commonly subject to.
**I will admit the it is curious how religious morality often appears to anticipate genuine scientific discoveries. For example, moralists have had in for smokers probably since tobacco was discovered to Westerners and it turns out in fact smoking is a bad idea for perfectly valid scientific reasons. In spite of this fact I am not prepared to defend groundless superstitions that infringe on liberty on the basis that those peddling them might accidentally be correct.
Peter