In my opinion, the idea of a monogamous marriage is more and more out of place in modern society. The economic argument for marriage is no longer valid. Divorce is now pretty easy, and most people have several partners over a lifetime.
I don't see any problem with people having more than one sexual partner (although this isn't something that I would want). And people can form emotional bonds with whomever they want.
What is the point of turning this into a marriage?