In France, any husband who can afford it has a mistress. Any wife who can arrange it has a lover. Everyone knows about it. No one cares. These are often lifelong relationships. Culturally different than the US but not as alien as Muslim polygamy.
Why shouldn't a man have two mistresses and no wife? A woman have two lovers and no husband?
In the United States we have begun to recognize a limited form of marriage called a domestic partnership. Like marriage a domestic partnership is a collection of rights and obligations. It is illegal to have more than one wife or husband. Is it illegal to have more than one domestic partner? Maybe it shouldn't be?