<p>Why do people cheat when divorce or consensual non-monogamy exist as more honest options? This article introduces the concept of the infidelity trap, a framework developed from interviews with men and women in long-term heterosexual marriages who engaged in infidelity. Participants described feeling caught between unmet needs and the emotional, financial, or social costs of leaving their marriages or the risks of suggesting renegotiation of them. Five interlocking constraints—compulsory monogamy, the overburdening of marriage, barriers to divorce, stigma surrounding consensual non-monogamy, and limited alternatives in the heterosexual dating market—converged to make infidelity appear as the only viable path. Infidelity, while morally uncomfortable, emerged as the only tolerable solution. The infidelity trap offers a new way to understand infidelity as a response to constrained agency, one shaped by monogamy’s cultural dominance, relational scripts, and the limited availability of viable alternatives.</p>