This article explores an Illinois domestic relations case in which a father (who had undergone sexual reassignment surgery to remove his internal female organs, and obtained an amended birth certificate changing his legal sex from female to male) was denied custody of the couple’s children based on the court’s ruling that the marriage was invalid. The court reasoned that because the father still had external female genitalia at the time of marriage, it was a same-sex marriage, and therefore invalid under Illinois law. The authors argue (consistent with a decision from the European Court of Human Rights) that “gender identity, public persona, hormone levels, and medical and psychiatric diagnoses” taken as a whole would be a better legal determinant of sex than “what physical genitalia a person has between his or her legs.” The authors further argue that “the law should not require complex surgery for a female-to-male transsexual.”