I feel that if you legalize any type of marriage other than man/woman you would then have to legalize any type of union between consenting adults. Cousins, siblings, parent/child over 18, polygamy, etc... Is that really where society wants to go? And while some of those examples seem just plain ridiculous so did gay marriage years ago....Be careful what you wish for!
Back to basic biology again-two men can't procreate, two women can't either. Seems pretty basic. Yes, there are all kinds of ways to have sex but that doesn't mean it was created to be that way.
And most of us man/woman supporters are not ignorant, etc...