"None of those are "good" reasons."
I think they're great reasons. I think equality is a good one too. And before you say "Equality! But..but...polygamists!!!!" remember that you admitted there were problems with polygamy. You found no problems with gay marriage, except that it might lead to polygamous marriages, which isn't a good reason, as far as I'm concerned. (I'm also realizing polygamy and polygamous aren't easy words to spell)