The last, worst argument against gay marriage
Most popular opposition to same sex marriage is driven by simple homophobia. I'm sure opponents would disagree, but it seems like the rapid shift in public opinion on the question is evidence that the once-prevalent attitude that gays are weird and/or gross is dissipating as straight Americans get to know more and more out LGBT Americans.
"Gays are weird and/or gross," though, is not a great legal argument, and right now the Supreme Court is deciding whether or not there is a good reason for the state to ban same-sex marriages while allowing opposite-sex marriages. So far the best that opponents of equality could come up with was "opposite marriage is just the way marriage has always been." That falls apart when you compare the modern American institution of marriage to marriages as they've been practiced throughout human history, as anyone who has read a book called "the Bible" could tell you. So the opponents moved on to their current argument -- the last, best argument for banning gay marriage -- which is, basically, "for the children."