Ampersand has been taking on some arguments against gay marriage recently, and in this post he makes a point that I've thought about posting (though unfortunately I've tended to forget about it once I get in front of a computer:

... many of my heterosexual friends have questioned if they'll get married or not because they're not sure they want to be part of an exclusionary and homophobic institution.

At the moment I think the number of people who would see marriage as delegitimized by including gays is greater than the number of people who see it as delegtimized by the exclusion of gays (especially considering that the anti-gay contingent is more likely to be part of a culture that sees marriage as a necessary milestone in life, whereas the pro-gay camp is more likely to see marriage as optional regardless of whether it's open to gays). But the cultural trend is headed in the right direction, and legalization of gay marriage would contribute to that movement, by creating "facts on the ground" that would influence those -- like myself before I got involved with punditry -- who haven't thought much about the issue.

But what I really wanted to post about comes in a comment on Amp's refutation of the anti-gay-marriage argument du jour -- "the purpose of marriage is to have and raise kids." JeffScott replies:

Actually, the primary reason marriage evolved, as is clear from the anthropological and ethnographic records, was to codify and institutionalize monogamy. And the reason for doing that was to minimize competition amoung men for access to women. Legal marriage historically has been an agreement between less powerful men and more powerful men, not between men and women.

I can't vouch for Jeff's anthropology -- indeed, it seems difficult to see how such a conclusion could be reached given that so far as I know, no societies without some sort of marriage institution have been documented. And his logic seems like it would apply just as well to polygamy as to monogamy. A more generalized form of the theory -- seeing marriage as a sort of social contract for sex, much as social contract theories typically deal with agreements about recognizing others' rights to property -- strikes me as having a degree of truth, though I haven't read much of the anthropological literature on the subject.

But let's consider for a moment the version Jeff gives, which specifies a contract among men to codify individuals' rights to sex. By that logic, it seems that those advocating fidelity to the orginal purpose of marriage would be forced to accept (indeed, even encourage) marriage of gay men, while banning lesbian marriage. When two men get married, they essentially take themselves out of the pool of potential competitors for women (not that they would be fighting very hard anyway, with the exception of bisexual men choosing a same-sex partner -- but even when purely homosexual men got married it would at least give some peace of mind to the remaining heteros so that they don't have to worry about those men deciding to enter the field). That makes the odds better for the remaining men. Lesbian marriage, on the other hand, would reduce the number of available women (if women even had the authority to make such a contract -- which seems dubious in the patriarchal situation Jeff indicates). The fact that these women may not have any interest in hetero marriage is moot given the reasonable assumption that in such a society a women could be forced to comply by her husband and father, or just by sheer economic necessity.