Cuz you know, nature and shit.

No marriage is bullshit invented by religion and slut shamers to try and guilt people into oppressing their sexuality.

That is why we have something that is called polygamy

You don't have to marry someone to stay committed and remain faithful to one person. If someone is openly polygamous and their partners are completely fine with that, knock yourself out. If you just want to remain true to yourself and not tie yourself to anyone, it's totally fine. However, most people in serious relationships and marriages aren't "open" because they consider each other an important part/extension of who they are. The "nature" stuff is totally bogus and ignores that you make choices of your own volition, probably comes from the same morons who try to deny evolution, and is just completely ignorant. You don't have to justify your preferences with a lame-ass excuse; the least you could do is say you don't want to be tied down to one person.

Many differently-phrased versions of this thread have been made before, by the way.

Some humans can be monogamous, some can't. Human beings are to complex of a species to say that whether of all of them can or cannot naturally commit to something like monogamy/marriage.

Like most things it's the development of society. Driven by religion or whatever. It's probably for the best with child support and all that stuff.