It's for this reason that I find the recent tendency of conservative Protestant leaders to entertain this argument disconcerting: it's disrespectful to their own traditions, and the authority on which it is to be accepted is fundamentally opposed to the usual sources of Protestant knowledge.
I want to add one small, limited point to Phoebe's discussion of contraception and abortion, which was raised in the comments by Britta but whose moment in that discussion has passed: it's true that religious arguments need not be of the no-abortions-ever-and-also-no-contraception variety. But, in fact, the truth is stronger than that: Roman Catholicism is (to my knowledge) the only Christian denomination that takes this position. Even in Orthodoxy, to the best of my understanding, the use of contraception is, if not entirely encouraged (certainly not in a premarital context), certainly not disallowed. The fact that Catholicism is the minority position even within Christianity is sometimes neglected in the rush to give attention to the Ross Douthats and Rick Santorums of the world.