How did one marriage paradigm supersede another? Carol Faulkner answers this important question in Unfaithful, a deeply researched and refreshingly original history of American marriage reform movements. Americans had long adhered to the English definition of marriage as a legal institution to organize reproduction and transmit wealth. Then, in the late nineteenth century, they came to believe that “only love makes marriage.” This narrative pays homage to Stephanie Coontz's 2006 grand narrative of “how love conquered marriage.” However, Faulkner uniquely attributes the rise of companionate marriage in the United States to “the nexus of the antislavery, moral reform, communitarian, spiritualist, free love, and feminist movements.” The ideological mechanism that made their rhetoric successful, she argues, was “the adultery metaphor.” Other historians have analyzed early critiques of marriage that compared the institution to slavery and prostitution. While these comparisons kept reformers mired in contract law and economic theory, radicals changed hearts and minds by comparing loveless marriage to adultery.Evangelical moral reformers first brought adultery to bear on marriage reform by criticizing the sexual double standard and advocating a wife's right to limit sex with her husband. Other reformers picked up this thread, suggesting that mutual love sanctified sex. If sex without love was nothing more than lust, then having sex with a legal spouse one did not love amounted to a kind of adultery—a spiritual betrayal of one's true soulmate. Thus, laws holding people in lifelong marriages contributed to social licentiousness. Working from this shared premise, radicals proposed divergent solutions: complex marriage in John Humphrey Noyes’ perfectionist community at Oneida, collective housekeeping in secular Fourierist phalanxes, the abolition of legal marriage to protect individual sovereignty, and open sexual experimentation among members of the Unitary Home in New York City.Social conservatives responded by branding all marriage critics “free lovers,” a slur which sex-baited moderate reformers into silence and divided the movement. Postbellum radicals doubled down. A small corps of shock troops flaunted their own adultery and publicized that of religious hypocrites. Here, Faulkner provides a fresh take on the Beecher-Tilton scandal of 1872. Faulkner interprets Victoria Woodhull's exposé of Henry Ward Beecher's adultery in defiance of Anthony Comstock's first obscenity law as part of a deliberate and orchestrated pattern of civil disobedience. The radicals soon dispersed under the weight of the repercussions for this strategy. Nevertheless, marriage critics won a larger cultural battle. The notion that love, not law, defined true marriage had become “normalized as marital orthodoxy” by the end of the decade. By making moderate legal reforms appear reasonable, even the most eccentric communitarians “were more successful than we think” in redefining marriage.Unfaithful abounds in archival detail, recovers stories that have been forgotten or minimized, and offers a persuasive narrative of change over time. Readers learn of couples such as George and Mary Cragin, whose evangelical impulse to discipline their idolatrous love for one another drew them to Noyes’ complex marriage scheme. We also discover that a free love movement which repeatedly defined true marriage as “an exclusive conjugal love between one man and one woman” could enable passionate affairs between women, such as Mathilde Anneke and Mary Booth.Faulkner's warts-and-all treatment of marriage reform is rigorous and reliable. Sensitive to the vulnerability of a dissident group, she assesses claims of sexual assault in free love communities and finds that some were fabricated but others all too real. Racism was the movement's most persistent flaw. Paschal Beverly Randolph, the most visible person of color affiliated with marriage reform, took the white spiritualist Andrew Jackson Davis to task for using the n-word while making a case that marriage was a more pressing national issue than slavery. Most white reformers simply presumed that their priorities mattered most, and their implicit bias ultimately drove Randolph away. Unfaithful presents a range of historical interactions with precision and care.The turn toward companionate marriage markedly improved the status of wives and expanded the framework of individual rights. Twentieth-century feminists carried out legal campaigns against marital rape and domestic violence, for no-contest divorce and maternal custody rights. Companionate ideology eventually made gay and lesbian marriage thinkable, even attainable.Yet the radicals who longed for the abolition of marriage might have seen liberalized marriage as cooptation rather than triumph. Marriage's affective premise made the institution more flexible and thus more durable. Even the expansion of divorce to include grounds such as alienated affection helped it thrive in an increasingly individualistic society. As Terra Hunter argues in Bound in Wedlock (2015), the postbellum redefinition of marriage as a civil right for formerly enslaved people increased its ideological and institutional power—often at the expense of other rights. Faulkner's narrative shows that the mainstreaming of companionate ideology also encouraged white Americans to associate marriage with the pursuit of happiness.Nineteenth-century radicals believed the freedom to love meant “social equality.” But while love took root in culture and law, equality did not prevail. Some queer critics raised this point during the celebratory aftermath of the Supreme Court's 2015 Obergefell v. Hodges ruling. Did love really “win” if spouses could still legally endure discrimination in housing, health care, employment, services, and civil society in twenty-nine states? Two years after the publication of Unfaithful, the conclusion that love has won seems unduly optimistic. In the wake of the Court's decision in Dobbs v. Jackson, it suddenly appears dangerous to consider prior rulings settled law. A more expansive version of love might win if the Senate passes the Equality Act—passed by the House and under consideration as I write this—which would make discrimination based on sexual orientation and gender identity illegal across the nation. However, as perceptions of an existential threat to gay and lesbian families intensify the emotional investment in marriage, defending the institution may strike activists as a more urgent goal. Will a future historian need to explain how marriage reconquered love?