Arguments against gay marriage are never really about gay marriage itself. Rather, they are really about fears of usurping gender roles, gender hierarchy and terror at the prospect of taking responsibility for one's sexual urges. Let me explain. You'll notice that the conversation focuses on same-sex relations between men, rather than between two women (other than to ask, "Who's the man?" and endless fascination about how sexual relations occur when there's no penis involved).
Judeo-Christian ethics hold that women are figuratively and literally underneath men; the idea of a man submitting to another man in that same way connects men in a very real way with sexual dominance -- it means losing privilege. Secondly, in the existent social hierachy, men ultimately are not held responsible for their sexual behavior. Allowances are made for having external genitalia that makes hiding one's attractions impossible. Women (and children) are given the role of policing male sexual behavior, domesticating unbridled male sexuality, to enforce social order. This order is so powerful that entire institutions like the Catholic Church have sprung up around it, portraying themselves as the voice of God.
What I know is this -- gay men are on the leading edge of taking responsibility for sexual behavior and honest negotiations for its meaning in committted relationships. This has been a hard-wrought phenomenon, all the more admirable for existing despite threat of incarceration during the AIDS crisis, religious abuse and social degradation. Straight men who are worried about how they'll possibly be able to remain faithful to their wives when mariage equality comes to pass would do well to start listening to gay men for help.
In fact, straight people who worry about Biblical admonishments against same-sex relations would do well to ask GLBT persons for assistance in this matter, as many of us have spent years in earnest contemplation on these matters.
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