Here is This American Life host Ira Glass talking on Savage Love about infidelity. Incidentally his mother is an expert in the field. But in any case it rings kinda true.
I cheated once when I was still at uni. We broke up (I never really liked him but my housemates did) and then he got some girl pregnant on some summer work thing in America and Wow. That worked out well. But I have been cheated on. Several times. When it mattered. And this is the most reasonable thing I have read on the subject.
“The fact is, marriage is the rules that you agree on… infidelity actually isn’t the sex, the infidelity is you’re confiding in somebody else besides your partner. Somebody else knows you better. Like, you trust somebody else more with the truth about you than you trust the person who you’re supposed to be closest to. That’s the real thing that actually rips people apart.”