Why is it illegal to have multiple spouses?

[Mister Agenda]

I think there should at least be an exception for immigrants who already have more than one spouse. Under current law, 'excess spouses' must be divorced; and it is particularly common among refugees fleeing war to get a divorce to qualify for living in the USA. That can be a hardship for the divorced spouses. They were part of a household, and were entitled to an inheritance. They might get left behind while the rest of their family goes to America. They might arrive and be in the position of a single parent trying to navigate a new country. I saw this sort of thing with the Somali Bantu refugees we got back around 2004.