Abstract: This Article questions whether and why it should be unconstitutional to treat legitimate and illegitimate children differently. It argues that legitimacy doctrine is rooted in a biological essentialism completely at odds with contemporary efforts to expand legal recognition of nontraditional parenting practices including same-sex parenting, single parenthood by choice, surrogacy, and sperm donation. The routine invocation of legitimacy doctrine by advocates purporting to help nontraditional families is thus at best ironic and at worst dangerous. Analysis of the U.S. Supreme Court’s legitimacy cases reveals that liberal Justices, in trying to dismantle marriage—a legal construct—as the arbiter of legitimate parenthood, presumed that a biological construct—genetics—was a superior arbiter. These Justices either did not realize or did not care that the biological determinism driving the impulse to protect illegitimate children could actually undermine a more progressive family law doctrine. Validating nontraditional family structures requires an embrace of law, not blood, as the arbiter of parenthood, and thus requires a very tempered reading of the legitimacy cases. Such a reading mandates that the state be consistent in how it determines parenthood, but does not require the state to recognize genetic parenthood. The power to define parenthood, which the conservative Justices felt comfortable leaving to the state, is best kept with the state, so that the law is able to break free from heteronormative family forms.