Reasons wh you want to have sex with several females

-you want variation.
-fresh young girls instead of being stuck with an aging hag.
-the genetic quality of your children will not depend on a single female.
-you will be able to have more kids.
-your wives will not be able to control you by refusing sex (assuming you are not willing to break the law and rape her).
-you will be able to handle breakups better since you ideally always have 2+ girlfriends.

If you get married and all that you are pinning everything on one female, not a good idea.