There's some other aspects, too -- one, the jealousy over sexual exclusivity may fade with time, or if it's not a significant source of person's sense of self-worth to begin with. And two, eventually we may come to realize that people will often lie in relationships about many things and doing it about sex with someone else is just one of those things. Requiring absolute trust, i.e. the partner always tell nothing but truth and absolute truth and be completely and utterly loyal 100% of the time, it's simply something that can't be realistically expected from anyone.