Everyone is debating about the whole concept of legal marriage and whatever. Who can marry who? Well, I've got the sensible solution, though it means shrinking the size of government and nobody would ever go for it.
It's this: do away with marriage licensing completely.
Yes, you read that right. I think the government has no right to say anything about who marries who and is not in a position to approve or disapprove of marriages. The reason marriage licenses are in place in the first hand is so the government gets more money to fund their dirty work. Of course, I can make a case for marriage licensing being unconstitutional, especially in the light of the fact that singles are in a higher tax bracket. This is unjust discrimination against singles based on their marital status, which is technically unconstitutional (but since when did the modern government give a rat's rear-end about the Constitution anyway?).
I'm no religious person (well, I'm a very religious atheistic Unitarian Universalist, but that's hardly being religious), but marriage in and of itself is indeed a religious institution, so let's just leave it to religion. Individual religious institutions can choose who they marry and what marriages they approve of. That way these religious groups can marry people according to their standards (there are some religions that do permit same-sex marriages and perform them in states where it's legal), so let gays go to churches/whatever that'll marry them.
In the end, you get to marry who you want, without government intervention, and this debate becomes a non-issue politically, but internally within each individual religion. Everybody's happy (except for maybe the government). End of story.