I believe that monogamy is a social construct. Humans are naturally polygamous, and the world is made up of either people that force monogamy on themselves (through religion, social pressures, self-imposed morals), or people that pursue their polygamous instinct (cheating on their SO, engaging in open relationships and other overt polygamous arrangements).

I think the proof of this is exceedingly simple: once you are in a forced monogamous relationship like an LTR or marriage, you don't just magically stop seeing other people as sexually attractive. If monogamy was natural, you would ONLY be sexually attracted to your partner. I believe that people who claim that they are not sexually attracted to anyone else while in a monogamous relationship are lying. They are experiencing these emotions/attractions, but are suppressing them because they are choosing monogamy over their polygamous instinct. And that's exactly my point; monogamy is a CHOICE, not a natural instinct. POLYGAMY is the natural instinct.