If Tarzan and Jane approach, and then join and are accepted by, a village, then they, as a couple, become subject to the way of life of that village.

In coalition terms, their little couple coalition becomes a part of a larger coalition, the village. Now there are roles and rules, and the concept of "fidelity" becomes applicable to the Tarzan / Jane (married) couple. But it is not fidelity to each other; it is fidelity to the village. Any roles or rules that are now applicable to them as a couple come from the way of life of the village, and their duty to conform is a duty, not to each other, but to the village.