Would a gay marriage ban be constitutional?

Allow me to play the devil's advocate for a bit. While I am fully opposed to gay marriage, I have to wonder if the Constitution could or should withstand a ban on same-sex marriage.
Is it the place of Christians to decide what marriage is? Is marriage a religious institution? If so, should should a union between two non-believers be called marriage? What about common-law marriages? Are they real marriages?
The more I look at the situation, the more I have to question the value of a same-sex marriage ban, especially one written into the Constitution of the United States. At face value, it's discriminatory against people who don't believe or behave the way the rest of us do. I think any time we infringe on the rights of one group of people, we infringe on the rights of all people.
What if the roles were reversed and there was a gay majority that wanted to define marriage for the rest of us? Where would we be then? Would that be right? Would it be fair?
While I feel homosexuality is unnatural, reprehensible and condemned by God, I don't think it's our place to force our beliefs and will upon those who believe otherwise. I think those people will have to answer to God, not a Senator, for their sin. As a Christian, I think it's our duty to discourage homosexual behavior and to teach the truth as God has shown us in the Bible. But as Americans, it's not our place to pass judgment on them or to tell them how they should live or whom they should love. I think the more we polarize and politicize the issue, the harder we make it to extend the love and grace of God to these people.
Anyway, that's some food for thought. Please feel free to reply and let me know what you think.