My ex did cheat, a lot. And he cheated on his fiancé before me, and the lady he married after me. (I never married him, I just knew he wasn't marriage material).
I strongly suspect my husband cheated (or at the very least had his head turned) 15 years ago, when he came in from a night out.
At the time, we had two babies, aged 5 months and 19 months, he had just lost his job (I wasn't working) and we lived over 1,000 miles from our family and friends. Needless to say, life wasn't much fun.
We almost separated, but due to finances, didn't. I do think he also did a mental stock check of what he stood to lose, and made a decision accordingly.
After we got through that, we became, and remain super strong. I doubt he has ever come close to crossing the line again. As he is a wonderful, loving husband and father, I haven't chosen to investigate my suspicions, as I really never felt this 'feeling' again.
And, no, he isn't a bad person. Quite the opposite. (And yes, I got myself checked out and it was an all clear)