To those of you who are opposed to gay marriage: why?

For the sake of playing devil's advocate, one argument can be that children will be better off when they have both a mother and a father. It can provide a sense of normalcy, teach them the experiences and strengths of both genders, and gives them a trusted adult they can relate to of their own gender (father if child is male, mother if child is female).

I think it's a weak argument personally, and studies are conflicted as to whether any of that is valid, but as far as secular arguments against gay marriage go, that's about as good as it's going to get