Note: The subject matter of this commentary may not be suitable for children. Please use parental discretion. Earlier this week, the cable network Bravo aired the documentary Three of Hearts: A Postmodern Family. The film is designed “to challenge conventional ideas of family,” according to director Susan Kaplan. Three of Hearts shows two gay men, Steven Margolin and Sam Cagnina, bringing a woman into their relationship. For several years, the director followed Steven and Sam—and the woman they called their “wife,” Samantha Singh. (Sam actually married her, presumably on behalf of both men.) We see them interact with their families,...