As the Supreme Court prepares to hear Hollingsworth v. Perry and United States v. Windsor, opponents of same-sex marriage have scrambled to answer the central question: What is the government’s rational interest in preventing gays from marrying? The standard argument from moral disapproval was revoked by Romer v. Evans and Lawrence v. Texas. The argument that gay marriages undermine the family has been debunked by a decade of same-sex marriage in several countries. So, as Proposition 8 and DOMA wound their way through the courts, gay marriage opponents lit upon a more durable argument, seemingly grounded in science rather than animus or religion. Their case, presented most comprehensively by Princeton professor Robert P. George, is that only sex acts with a ‘dynamism toward reproduction