I agree with Pen3 OP MC usually have that in those ancient fantasy. They aren't christian after all.
Fact : Monogamy was instaured as only choice in the council of trent around 1560 before they can have multiple wifes. They have penalties before that(around the 13th century "1215"
so only western civilisation was hit by that.
Source http://www.restud.com/wp-content/uploads/2014/12/ms17025-paper.pdf
Now you will sleep less dumb and learned something.