Why is it illegal to have multiple spouses?

[Anomalocaris]

So why am I not permitted to have 1 wife and 5 husbands?

for patriarchal societies,  it might be much less serious threat to social order for a man to have 2 husbands, but it would be for a woman to have 2 husbands.

the prohibition against polygamous homosexual marriage arose out of much later porting over of the form heterosexual marriage strictures to homosexual marriages