This may be the only thing I agree with Andrew Sullivan on, but it's worth a read, particularly if you wanna through it back in the face of the likes of Pat Buchanan, with his own terminology and understanding about marriage.
The main crux is that, since Pat Buchanan doesn't have kids, unless he and his wife are totally celibate, then they are not having sex that has been sanctioned by God. This means that either Buchanan and his wife are using birth control, or one of them is sterile. Sullivan points out that the Church has been forgiving of sterile hetero couples, allowing them to marry even though there is obviously no procreative purpose for their union.
At any rate, it's a fun read for those of you who might be looking for a way to tackle gay marriage from a religious point of view (or, in their own terms, as it were).