The concept of marriage was created because ancient humans understood that children do much better when raised by two parents rather than one, and they wanted to coerce couples into staying together. If everyone just goes whoring around, randomly producing children who have no idea who their father is, the society quickly crumbles. We're watching it happen right now.

It's manipulation, sure, but it's a necessary one. You can't just say "you have to stay together because it's the right thing to do", because ultimately very few people actually care about morality. The only way to get most people to behave is to threaten them with some dire consequence.