My main argument in favour of gay marriage is, why should I care? If two (or more) consenting adults want to make a lifetime committment to each other, why would it bother me? I'm not affected at all.
But clearly there's a lot of people who don't feel that way, and they're probably not all insane or stupid. So what is the argument against gat marriage?
I've done some reading, and some thinking. Suppose you're convinced that:
1) The God of the Hebrews exists
2) He has forbidden gay marriage (Leviticus, etc).
3) If you don't do what he says, he'll commit genocide (there's a few instances supporting that).
Then if a marriage happens that is contrary to what the God of the Hebrews allows, he might kill us all. That is, obviously, a rational reason to be against gay marriage.
But then I thought some more. It's contrary to the law of the God of the Hebrews, to wear clothes of mixed fibres, sow mixed crops in a field, or eat from a fairly long list of forbidden food. And presumably he still has the genocide deterrent to discourage these activities.
So I can understand why some people would, rationally, be against gay marriage, but then I can't understand why they eat bacon sandwiches.