This can be addressed by many different standpoints...
Polygamy was once a societal norm in ancient times
for example, the more wives a man had, the more powerful he was perceived
Cultural: Harem etc
Western Religious: Catholic marriage is a sacrament yet the CofE adopted divorce, therefore devaluing the sacrament of the vow of monogamy.
In contemporary times, Hollywood placed an emphasis on love stories & romantic comedies, even Disney made us believe in "happy ever after"
From a personal standpoint, this friend with benefits and Netflix and chill culture hasn't helped the situation if you're trying to be monogamous.
Technology is a blessing and a curse: Online dating & hook up sites make it easier to "cheat"
Take into account the type of girls they are, their age, their sense of entitlement and privilege, their integrity... Same for the opposite sex.
Thing is, morally our society acts so scandalised when someone is outed as a cheat but these same hypocrites end up doing it themselves!
People have all sorts of reasons for doing it from just being selfish to needing the attention and validation. Some are missing excitement and passion, some seek out a side person to explore sexual fantasy, some are oversexed and unsatisfied...
It's easy to say if you're not happy in a relationship then leave yet people panic, maybe through fear of being alone or maybe because there are children involved.
I've been on both sides of the fence and I can't explain the emotional rollercoaster of either end of the spectrum.
If you're so disillusioned that suggests to me that you're a good one! Whilst you may open up yourself to hurt or being used remember life is too short so don't rule out anything based on a what if.
Hope that helps a little...