Over at Philosophical Disquisitions, John Danaher is discussing the "Polygamy, Incest, Bestiality" (PIB) slippery slope objection to homosexuality. He reviews several attempts to counter this argument by describing how homosexual activity is different from PIB, and (paraphrasing John Corvino) rightly points out that there is logically no need to make such arguments because the people who equate homosexuality to PIB do not provide any argument supporting that position (the only similarity being that they are all "forbidden" or "gross").
Along those lines, the PIB argument really isn't an argument in itself; it seems to be a reductio ad absurdum for some unspecified justification for acceptance of homosexuality. The logic seems to be:
1) You say that we should accept homosexuality because of principle X.
2) Principle X would cause us to accept PIB (or pedophilia) .
3) Therefore, principle X is invalid.
Of course, the force of this argument relies entirely on what "principle X" actually is; if principle X does not apply to PIB, then the PIB-argument is pointless.
So this brings us to my favorite "principle X" : absolute sexual equality.
My principle is that I should not treat a person differently due solely to their biological sex (aside from my own sexual relationships). I realized that this principle applies to homosexuality when I read that some opponents of the sexual Equal Rights Amendment had predicted that the ERA would forbid discrimination against homosexual relationships. Many proponents of the ERA reject this interpretation. I'm not a lawyer, so I can't say anything about that -- but I do agree that the principle of sexual non-discrimination implies acceptance of homosexuality.
Attempts to distinguish homophobia from sexism often rest on the idea of reciprocal equality: that discrimination against homosexuality will limit a man's options just as much as it would limit a woman's options-- both men and women would be limited to sexual relationships with the opposite sex (let's ignore the intersex in this argument).
Superficially, this argument seems reasonable, but for any American it should quickly raise a red-flag, since it is reminiscent of the principle of "separate but equal". Based on history, we know that an arrangement that is superficially egalitarian can act as a cover for severe oppression. Based on logic, we know that when we treat people differently, it is very difficult to perfectly balance the moral worth of those differences; this is especially true in a world where people have different opinions regarding the moral worth of different conditions. In essence, requiring that men marry women and women marry men is the same as requiring that men work outside of the home and women work inside the home. We may believe that we can put aside our personal preferences and assert than marrying a man is objectively just as good as marrying a woman. However, as with a belief that working inside the home is just as good as working outside the home, this is actually a personal preference and not an objective description of reality. Furthermore, we cannot give the benefit of the doubt to these assertions of equal value, because such arrangements are quite easy to manipulate in favor of one group or the other (as we have seen in history).
For the sake of illustration, consider these two ways in which this "reciprocal equality" could lead to clearly non-equal outcomes. These may seem silly on their own, but they are sufficient to demonstrate the inequality implicit in considering sex when judging relationships.
The first inequality arises from the physical and emotional differences between men and women. For instance, men (as a group) are stronger than women (as a group); if we were to forbid a person from choosing a partner from either group, then we clearly would be influencing the likely characteristics of the partner that they eventually find.
The second aspect of inequality is apparent when the population is divided into two "reciprocal" groups that vary drastically in size. Consider splitting the human population into two groups, each of which is forbidden from marrying within its own group. Now imagine that one group constitutes 90% of the population, and the other is 10%; the 10% is clearly the elite group here, having their choice of partners from the other group, whereas most of the members of the larger group will be effectively forbidden from marrying at all (unless the members of the smaller group can take multiple spouses). Normally, the population of males and females is nearly equal, so this isn't a big deal, but this thought experiment does emphasize the fact that men and women are selecting their partners from different groups, competing for attention with a different group of people, and that these group dynamics will influence the likely outcome of their quests to find partners. To bring this back to the real world, some societies have experienced substantial imbalances between the male and female population -- a deficit of males following major wars, and a deficit of females is societies that practice prenatal sex selection.
Now that I've stated my position, let's consider how it compares to the position of others. Many people consider themselves "anti-sexist", yet would balk at taking this position of non-discrimination between homosexual and heterosexual couples. Looking at the marriage issue and their behavior in general, they apparently think that discrimination is quite appropriate; they actually are sexist, they only object to the idea that one sex is superior to the other. They will treat men and women differently in everyday encounters and believe that it is totally just for the law to recognize these categories for issues such as marriage. My objection to such categorization is both moral and empirical. Morally, individuals should be free to structure their lives as they see fit, and pigeonholing a person into a gender role places a fairly arbitrary restriction on their freedom. Empirically, many people lack the stereotypical traits of their gender on which theories of sexual discrimination are based (e.g. many men have less strength than the median strength among women). Trying to force a person into a role based on their genitalia is a simplistic approach that ignores the many ways in which each person deviates from any sexual ideal.
On the other side, there may be people who consider my view of sexual equality to be limited, in that I do consider there to be difference between men and women. These differences are apparent both in how I relate to them sexually (which only applies to my wife now) and that the male and female populations differ in many socially important respects. I won't deny that our sexual preferences and relationships have myriad ramifications in other aspects of society (thereby undermining the idea of sexual non-discrimination); however, by recognizing that the male and female populations are in fact different, I do not automatically assume that any difference in social/economic standing is due to discrimination. There is simply no getting around the fact that people are different from each other, and the details of how these differences arise (e.g. genetic, chemical, social) are not really important. We cannot and should not treat each individual identically, all we can do is make sure that our discrimination is not based on unsound ideology and prejudice.
So to bring this back to the issue of social acceptance of homosexuality and the "PIB objection", I think that it is immediately clear that this principle of equality does not compel us to accept polygamy, pedophilia, or bestiality. A variant of this principle (non-discrimination on the basis of parentage) could be used to argue that incest should not be discouraged. Of course, this would in no way affect the situations where incest could be viewed as rape (whether statutory or forcible). I'm not too concerned by this use of the argument to destigmatize incest,partly because I don't think that cousin marriage needs to be stigmatized, and so this issue is limited to sibling marriage. More importantly, most of these arguments for sexual equality do not apply strongly to incest -- primarily because the the prohibition of incestuous sexual activity does not place a substantial restriction on a person's choice of mates.
The advancement of sexual equality is one of the major struggles of our age, and no attempt to equate it with sexual perversion should distract us from eliminating sexual discrimination from our institutions.