One of the counter-arguments I've heard most from anti-marriage equality folks is that there is no right to gay marriage in the Constitution. They say this rather triumphantly, which is rather odd. I mean, it's kind of plain to me as a case of equal treatment under the law. Straight couples get scads of legal rights, benefits and responsibilities by getting married, and gay couples didn't. That's pretty simple.
But if that doesn't convince you, this clip is extremely instructive, and not just a little entertaining. This guy knows his stuff, speaks clearly and concisely, and would be very, very difficult to argue with.