Interesting qustion. I do believe we have to establish ethics that we all agree to live by.
The question is what shoudl our ethics be here. The question most often being asked is whether or same sex marriage is against natural law. Well, marriage is a man made institution, so not natural. However, as some have pointed out, homosexual activitity occurs in nature, so it is natural by definition.
I suspect if we are going to say in a free society that this group of people can't marry, we should present a reason as to why not. I don't think it should be religious because we are free in this country not to believe (though I question whether God has an objection or not), and I don't think tradition is enough (tradition was against inner racial marriage). But we should have a reason.
I can't think of a logical objection to same sex marriage; therefore, ethically, I'd have to support it.