Some people like monogamy and marriage because it makes them feel safe. Their partner is not allowed to leave them, and not even allowed to look around for something better. And, if the partner does anyway, these people can now play victim, and most everyone will agree they were wronged. This is a bit perverse. For one thing, it seems to say "I'd rather you stay with me, than be as happy as possible, because I'm just that selfish."