Why Are Homosexuals Against Polygamy?

It's the authoritarian mindset of the right.

To the right, morals are not a matter of sitting down and trying to work out the best rules to have to allow the greatest liberty with the least pain. Rather, morality is a set of rules that you are handed and which you then must obey without question simple because... well, they are the rules.

So to the left, gay marriage is modifying an existing moral/legal rule into a better one. But to the right, it's rejecting the authority underlying the rule. And in their minds, if you reject the authority behind one rule, why wouldn't you reject all rules? They see it like a parent whose kid refuses to clean his room and gets away with it - what will he refuse to do next once he knows he can disobey his parents?

So in their minds, they genuinely can't see the difference between allowing gays to marry and marrying goats or helicopters or rocks. People are either following the rules of marriage as the right (wrongly, as it happens) believe them to have always existed, or you may as well let anybody do anything.

Of course they don't want to admit this, even to themselves. Which is why during the huge national conversation America had on this subject, not one person was ever able to articulate a solid, sensible reason why gays shouldn't be able to marry. Because their real answer is "But that's naughty!" and they know they'd look like idiots if they said that.