Because gay couples gladly adopt the children that heterosexuals who can't stop humping each other don't want? Either homosexuals need the same benefits so they aren't disadvantaged, or we need something to prevent those heterosexuals from multiplying like crazy.

Besides, marriage isn't and never was about "helping couples reproduce". Even if it was, it's pretty clear that they don't need help. Seriously. Abortion is one of the main political issues today, do you really think they wouldn't manage to reproduce without marriage?