I can't remember where I read it (or heard it? podcast?) but there was a tribe in South America where children had multiple fathers. The women would have sex with whoever they wanted and when she got preggers all of the men who had sex with her become the fathers of the child. They believed that the semen was like a big brew that helped bring the kid into the world.
I've never understood the attraction of marriage. I also have many friends either divorced or in incredibly unhappy marriages. In fact I find it hard to think of any couple I know who aren't sacrificing happiness to stay together with their partners. Maybe it's just the people I know...