I don't see 'having sex with other people' as by definition unfaithful. I think it depends on the characters, the relationship(s) they have, the expectations they have of themselves and others and the circumstances they live in. But then, I believe in polyamory too :-). Even with marriage it is going to depend on what it was the people actually swore to each other.

I, personally, have an embarrasment squick, and I really don't like reading the parts where characters I care about are being Really Stupid (Miles Vorkosigan at the beginning of 'Memory') though I like the recovering from the consequences part. (I really don't like suspense, either -- it makes me feel physically ill -- so 'waiting for the other shoe to drop' is not my favorite part of that scenario either.)