Why is it illegal to have multiple spouses?

[Anomalocaris]

So why am I not permitted 1 wife and 5 husbands in any nation?

Social order seems an unlikely motivation against its prohibition given the many things we allow that disturb the social order.

I can cohabit with a thousand men and women and spend all day making pornography and selling alcohol and printing subversive political tracts and blaspheming all religions and all kinds of other order-disturbing stuff.

But I can't marry two people of any gender.

Doesn't add up.  Smells like religious influence.

even religious influence has its roots in the social structure and social problems needing solution.