[mikeash]

It's not "begging the question", it's simple logic.

If Elsa can marry Joe, but Steve cannot marry Joe, and this is purely because Elsa is a woman and Steve is a man, then you're prohibiting Steve from doing something purely because of his sex. Such discrimination based on a person's sex is wrong.

Really, sexual orientation is a red herring in this debate, but people can't properly break the situation down. Bans on "gay marriage" take rights away from everyone, not just homosexuals; although I may never exercise my right to marry a man, I should still have that right.