You can't objectively label different cultures as not being advanced just because they're different. There could well be good reasons for enforcing monogamy on people that your society disregards. I'm just guessing here, but maybe no monogamy may end up with a lot of people suffering the natural human emotion of jealousy and perhaps something like most women queuing up to be with the most attractive men, leaving the majority of men without sex their whole lives. Maybe ancient societies experienced problems like that and decided to enforce a more "wealth distribution" kind of system like monogamy as a solution. Modern societies often laugh that off but they also can't explain the reason for it. It's a bit like deleting ugly code when you don't realize it prevents a corner case bug.

If we really had complete sexual freedom and didn't impose legal restrictions on it to protect human emotion then rape would be nothing worse than other kinds of physical force, even for children. Maybe that's how an even more advanced would see it?