I don't believe in the "once a cheat, always a cheat" trope that is often rolled out on here.
It just makes some people who have been cheated on feel better about themselves: it means it definitely wasn't anything to do with them...
However, I would never even entertain the idea of forgiving someone. The only way I could forgive it is if I wasn't that emotionally invested/committed to the relationship myself.
I couldn't imagine being intimate (I mean generally, not using it as a euphemism for sex), or vulnerable, or laughing, or trusting or giving myself completely (again, not a euphemism for sex) with someone who had shown me such disregard, lack of consideration, lack of respect.
There comes a point in an interaction with someone where a person realises they have a connection. At that point, a person makes a choice. A choice to be loyal, a choice to respect, a choice to love their partner. Or a choice to think "fuck it, why not". It is that moment of choice that I couldn't forgive.
It would be less about "he had an orgasm provided by someone else" or "he has touched another woman's breasts" or "he was sexually aroused by someone other than me" because, whilst none of that is nice, that's not the bit that destroys the relationship and the trust. It would be more that, that at that moment, when they had a choice, they did not choose me.
If someone did not choose me, then I would not choose them.