Don Browning and Elizabeth Marquardt's essay in The Meaning of Marriage offers some "liberal cautions on same-sex marriage."
In the discussion of sex, it is normal now for liberals to automatically translate that term into "gender." The prevailing liberal theory is that no feature of masculinity or femininity is rooted in the biology of men and women. Everything is a social construction.
Everything, that is, except sexual orientation. When it comes to sexual orientation, liberals become essentialists again. Advocates of homosexuality are increasingly drawn to the idea that sexual orientation is a biological fact, not a social construction (and certainly not a choice). Therefore, society must accommodate to all sexual orientations and "get used to it."
Browning and Marquardt point out that the logic of modernization is dissolving all claims of fixed relations and innate or essential qualities. Even the deepest and most ancient of essential differences, those between fathers and mothers in marriage, are subject to the acids of social constructivism. It may be premature to fix the flag of same-sex marriage to homosexual essentialism today; the same modernizing theory that all identities are just social constructions may sweep it away tomorrow.