Adultery breaks a promise that was freely made

The argument

Proponents of this position begin with the nature of the commitment itself. A marriage or an exclusive partnership is not an arrangement imposed on anyone; it is a promise offered freely, in full knowledge of what it rules out, and usually made in front of the people whose regard both partners care about. Nobody is obliged to make such a promise. Having made it, on this view, a person is bound by it in the ordinary way that promises bind, and adultery is the clearest case of breaking it. The argument is therefore not primarily about sex. It is about what a promise is for. The point of promising exclusivity is to give the other person something they can rely on while they build a life that assumes it: shared money, shared children, shared plans, a settled sense of where they stand. Each of those decisions is made on the strength of the promise. To break it secretly is to let a partner go on building on a foundation the promiser knows has gone, which is why defenders of this position describe adultery as a wrong done to the partner rather than a private indulgence. Proponents also argue that the freedom to make the promise is what makes breaking it indefensible. If the terms had become intolerable, honest courses were open: say so, renegotiate them, or end the relationship and accept the cost of ending it. Adultery takes none of those routes. It keeps the benefits of the promise, including the trust, the standing and the shared household, while quietly abandoning the obligation that earned them. On this account what makes adultery wrong is not desire, which nobody chooses, but the decision to break a commitment while continuing to accept everything it secured.

Premises

Counter-arguments

Critics answer that promises about feeling are not like promises about conduct, and that a vow of lifelong exclusivity binds a person to something they cannot entirely control. On this view the promise is better read as a statement of intent than as a contract, and holding someone to it decades later, under circumstances neither partner imagined, is unreasonable. Others accept that promise-breaking matters but deny that it settles the question: what weight the breach carries depends on how the relationship had actually been functioning, on whether the terms had become a formality, and on whether the partner had already withdrawn from them in other ways. A broken promise, they argue, is a reason for criticism rather than a verdict.

Rejecting the premises

[Rejecting P1] Critics reply that the promise is rarely as free as this makes it sound, since social and family expectation shape whether it is made at all and what terms are even on offer. [Rejecting P2] They add that reliance cuts both ways, because a partner who has long withdrawn from the relationship has also stopped honouring what the other relied on. [Rejecting P3] They argue that the honest alternatives are far costlier than this suggests, since ending a shared household can harm the partner more than the breach does. [Rejecting C] They conclude that the broken promise is one consideration among several rather than something that makes adultery wrong on its own.