Here's a link to a paper by two political scientists and Princeton philosopher arguing for an anti-gay conception of marriage.
Over at Philosophy, Etc., the refutation is well-argued.
I don't think I have anything to add except one fact. The natural view of marriage as facilitating human reproduction is argued as a "metaphysical fact." The insight shared by French feminists, post-structural theorists and thinkers like Levinas is clearly suspicious of Western metaphysics for tending to promote discourses that subsume all difference into a category propped up and passed off as metaphysically real. Such suspicions drive at the heart of thinking women are lesser than men due to apparent metaphysical arguments just as much as systemic discrimination against Blacks in the American South was propped up by phrenology. This is why discourses of the "Other" are so pertinent, and cannot be ignored.