If you are really interested in learning more about infidelity I would suggest you read / listen to Dan Savages's columns / podcast. Unsurprisingly there are a lot of different calls and discussions about fidelity and monogamy. I would also suggest you read Ester Perel groundbreaking novel The State of Affairs: Rethinking Infidelity

To answer your question directly infidelities are fairly common because monogamy is difficult and society typically puts a lot of stress and pressure on monogamy that makes it even more difficult. The foremost is the idea that monogamy as a default setting and that one person can be everything for one person sexually, emotionally etc.. So we have a situation where two people are assuming that their partner will be the only person they can be attracted to, the one person that can fulfill all their emotional needs and will have to be a perfect roommate / life mate. This coupled with the idea that you have to be perfect at monogamy or you are a complete failure at it. So you have a hard situation and hard expectations so people slip and some people who slip on something small (a micro-cheating which is a ridiculous concept) they go all the way. All these expectations are common in "Straight-land" while those in "queer-land" have a different set of expectation that work better for everyone.

We should all make monogamy an regular opt-in conversation for relationships (I would suggest ~6 months in when you go exclusive and then at most every 5 years). Moreover, we should understand our monogamist partner finds other people attractive and chooses to not pursue anything not that they don't find anyone else attractive. Finally, we should understand a our partner needs friends, hobbies and confidant who are not us to rely on. Until that happens "cheating" will likely remain something that happens fairly regularity