I don't think the full faith and credit clause is going to allow room for that. Also, the Constitution is the device for laying down nationwide rules when clarification is needed: Defining marriage and the National Language are two good candidates for this...
But that's just the point. I don't really think that "defining marriage" is a duty of ANY level of the government (whether state or federal), because I see marriage as a religious, rather than secular issue. Unfortunately, we in America have so secularized marriage that it can be performed outside of ANY church or religion (by government officials) and valuable and important sections of our tax code are based around the concept of marriage.
I don't think that marriage was EVER meant to be anything but the union of a man and a woman under God, but if we've already bastardized the act by intermingling it with national law and the like, we might as well leave this "issue" to the states (by referendum), rather than the out-of-touch Washington crowd. What I presented in my previous post is the compromise of my actual beliefs, having realized that by the courts (left) or the legislature (right), America will overstep it's boundaries and order the churches of America either to marry/refuse to marry homos.
Did that clear anything up?