Yesterday's Jesus and Mo strip, in addition to making me chuckle, also gave a rationale for supporting gay marriage that roughly summarized something I wrote years ago:

Let's look at a case of moral reasoning which has to do with purely heterosexual behaviour; namely an extramarital sexual relationship between a man and a woman. All Muslims would of course agree that this is immoral in principle; but now let's refine our view of it. Suppose there were two such couples, one of which was monogamous and one of which was not (ie., either the man or woman or both are also involved with other people at the same time).