There is some highly questionable decision-making going on here, to the point where your original question is just about your least important dilemma.
Your boyfriend cheated on you. That means he was sleeping with someone else, returning to you, and lying about his whereabouts. For months.
Then he broke up with you.
To me, those two things tell me far more about him than the two years since.
The only time a partner has any say over your sex life is when you are dating exclusively.
Your boyfriend is a liar. Your boyfriend is a cheater. Your boyfriend felt that his life would be enhanced if he were no longer dating you.
You slept with a guy on break. There is nothing unethical about that — especially since it wasn’t a “break” at the time. You were single. Single people are allowed to do whatever they want. As such, you should have absolutely no guilt about your actions, nor should you feel compelled to tell him about them.
Do you think your boyfriend was celibate during your break? Does it matter who he slept with when he was not your boyfriend? I sure don’t think so. The only time a partner has any say over your sex life is when you are dating exclusively.
Which brings us back to your current boyfriend of two years, who has been known to date you exclusively and still sleep with other women behind your back.
If you can forgive his crime, he should have zero to say about what you did when you were single.
You can already tell my feelings about him, and understand why I’m dubious about his ability or desire to make you feel safe in your relationship.
But ultimately, you have to make the call as to whether his cheating spree (it’s not an incident if it happened over and over again) was anomalous.
All I can say definitively is that if you can forgive his crime, he should have zero to say about what you did when you were single.