//dusts off history degree//
The Victorians were actually a bunch of non-married living-in-sin rutting yahoos. Go look at the birth records for the lower classes. In fact, that's true throughout European history. What we think of when we think of Victorian sexual repression is the behavior of the upper-middle, upper, and aristocratic classes.
Also, momogamy is rare in nature. How do we know we're designed to be monogamous? That might just be someone's church forcing their dogma on you! (Kind of like the Pope not letting people wear condoms to protect from AIDS. That makes my head turn around like Linda Blair's.)
What usually works best in such situations is the majority of people doing what feels morally correct to them, and then doing their utmost to mind their own business when it comes to the behavior of others.