Criminal adultery bans, despite widespread transgression and lax enforcement, remain on the books in a substantial minority of states. The landmark Lawrence v. Texas decision casts doubt on all state interference with consensual sexual activity among adults, including adultery bans. Additionally, adultery bans on their face implicate the Establishment Clause, due to adultery bans’ and marriage’s roots in religious doctrine and religiosity. This Note examines the constitutionality of adultery bans after Lawrence v. Texas, and proposes a novel approach to substantive due process analysis that applies Establishment Clause values. In proposing what this Note dubs the “Establishment Clause prism,” through which a facially legitimate state interest is delegitimized if substantially motivated by religious forces, this Note concludes that adultery bans are unconstitutional.