It's not even just the sex. It's just unrealistic that 1 person can meet all of your needs forever and you can meet all of theirs. So you accept that you compromise on however much stuff...your partner doesn't make you laugh as much, or their taste in music is a bit crap, or you fight over how childcare or chores or something would be and so you end up seeking what they lack in other people. And I'm quite selfish and want absolutely everything to be wonderful and also want my husband to be perfectly happy and it's impossible. Monogamy is just mental.
True. We emotionally cheat on each other all the time. Where we realize "we can't talk about ______ with [partner]" so we go to other people .Why is it different for sex?