Possibly more realistically, the person may realize - from observing the world - that the only way he or she will be able to maintain monogamy is through social (not just government) enforcement of the marriage contract - not that his or her life literally depends on it, but that his or her social death will result from violation of the contract. And people care a whole lot about social death. This aspect of social support of marriage is already gone from all but a few recent immigrant communities in the United States. Even if marriage were government-enforced for reals, collusion (pretending grounds for divorce existed) and stretched notions of "cruelty" were already common before no-fault swept the nation. The government maybe slowly changes its enforcement toward the enforcement of whatever limping modern non-tribal community happens to exist.

Anyway. People are sometimes harmed by getting extra choices. And people are sometimes harmed by losing choices.