What counts as a breach depends on what the partners actually agreed

The argument

Proponents of this position start by separating the moral question from an inherited default. Exclusivity is a term some couples adopt and others do not, and on this view its authority comes from the agreement rather than from the institution. Where two people have genuinely settled on exclusivity, breaking it is a broken agreement and can be criticised as such. Where they have not, whether because the arrangement is open, because the terms were never discussed, or because both have long understood the relationship differently from the way it is described to others, there is no breach to condemn, since there was no promise of that kind to break. The argument draws its force from how varied actual relationships are. Couples arrive at understandings that bear little resemblance to the standard script: arrangements that permit relationships outside the pair, long separations with terms attached, partnerships that have become companionate by mutual consent. Proponents argue that a rule condemning adultery as such cannot distinguish any of these from secret betrayal, and so condemns people who have wronged nobody. What matters morally, on this account, is whether someone has been deceived or treated unfairly, and that is a question about a particular relationship rather than about a category of act. Defenders of the position also note what they take to be the source of the opposing view: it borrows its authority from a tradition that treated marriage as a matter of property and lineage, and keeps the prohibition after that rationale has gone. Strip the inheritance away, they argue, and nothing is left but consent, honesty and harm, all of which are already covered by the ordinary moral rules applying to everything else people do. On this view adultery is not a separate wrong at all; it is a label for one way of mistreating a partner, and it applies only where a partner has actually been mistreated.

Premises

Counter-arguments

Critics reply that this understates how much is settled without being spoken. Marriage carries an understood default of exclusivity, and someone who marries without disclaiming it has in effect accepted it; claiming afterwards that the terms were never agreed looks to them like a convenient reconstruction. They also argue that the appeal to open arrangements is beside the point, since the cases that provoke the question are precisely those where a partner did not agree and was never asked. On that reading the position is unobjectionable where it is true and irrelevant where the question actually bites, because almost every disputed instance is one of secrecy rather than negotiated latitude.

Rejecting the premises

[Rejecting P1] Critics object that exclusivity is a social understanding as well as a private agreement, so entering a marriage without disclaiming it amounts to accepting it. [Rejecting P2] They argue that the existence of unconventional arrangements says nothing about the ordinary case, where no such latitude was ever discussed. [Rejecting P3] They reply that silence on the terms is not the same as the absence of terms, since the default was understood by both. [Rejecting C] They conclude that the position excuses only the cases nobody disputes while leaving secret infidelity exactly where it was.