Getting back to the question that started the thread--
Gay marriage is linked to anal sex and many people find anal sex disgusting. Exhibit A: Martin Gibson.
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Martha C. Nussbaum [the Ernst Freund Distinguished Service Professor of Law and Ethics at the University of Chicago,] said "Disgust is never a good legal argument." Her dispute with disgust rests largely on psychological evidence. She cites the work of the experimental psychologist Paul Rozin, who, after testing people's reactions to different disgusting substances, concluded that the core objects of disgust — feces, spit and semen — are all things that remind people of their own animality and decay. Disgust is revulsion at the prospect of taking decaying animal products into the body, she says. It guards the body's borders.
... The problem, Ms. Nussbaum says, is that many things seem disgusting only by association. People won't eat chocolate that has been molded to look like dog doo, or soup served from a sterilized bedpan. More to the point, people learn to hate by linking their natural sense of disgust to certain people. Misogyny, she argues, is based on the idea of women as sewers, receptacles for sperm.
http://www.truthinjustice.org/emotion-law.htm
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My second point-- The bible never explicitly states that homosexuality is wrong. You can go through the Bible with a fine tooth comb, and cull out a few passages. But when you go back to the original Greek or Aramaic, the word that is used means "temple prostitute." So the patriarchs were saying, People should not have sex with whores [of both sexes] at the temples.   

I have a wonder ful cartoon of angry micro-organisms gathered round a bride and groom micro-organism. They are shouting, "You are defying millennnia of single-cell reproduction!"

In geological time, or biological time, the insitution of Western European nuclear families is just a blink.