there is nothing immutable (or in any way natural) about marriage. It's a human-created concept based on nothing beyond ownership of other humans as property. Literally, that is why marriage exists. Nothing else.
Naturally, our species really hasn't evolved in a primarily monogamous mating strategy. "Community" is a better definition, because children can, and in many communities across the world, raised that way. Doesn't mean it works that way for everyone--do what you prefer and are happiest with--but arguing that this is natural for humans is profoundly naive, and reveals nothing but plain ignorance regarding actual human biology.