The simple truth about cheating is that, as a species, we aren't designed for long-term monogamy (of course, since we're all different, it may work for somebody).

That's why most people will eventually cheat in their lifetime (*). For a thousands reasons. And cheating doesn't mean s/he is a horrible person you can never trust: everyone is different; everybody makes mistakes.

I have been the "side guy" for some wives who were really unhappy in their marriages, and our relationship helped them to clear their mind about it.

Personally, I distrust those who say "I could never ever cheat". Most of the time they're just naive, or not really honest with themselves.

(*) If you Google "how many people cheat", you'll find many studies that support my thesis. The first page reports at least 71% of men and 82% of women admitting cheating: https://techreport.com/statistics/lifestyle/cheating-statistics/

Any research reports different numbers, but most go over the 50% threshold.