Little relationship problem here..

The trait won’t make them go out and cheat. But it will make (or should make) it more difficult for romantic advances to be accepted, meaning your Sim will have to try a little harder when it comes to that first kiss, asking them out, and eventually popping the question.
And even then, he’ll get a tense buff every once in awhile from being noncommittal and in a relationship for a long time.