How common is it that you'll cheat back?

In my relationship my partner has been through some incidents he feared I would consider cheating and the way he handled them, out of fear, caused a trust issue, not the incidents themselves. He said he feared I was going to leave him or I was going to cheat but still remain in the relationship, as I was not leaving his fear then grew that I was going to cheat, that I had a form of right to cheat, thoughts, feelings about this got difficult for him to handle over time. I would not cheat "back", if anything I would leave if I thought that and to me these incidents were handled somewhat too late then it needed to be, but still handled the right way, finally, and in our past, I then just let them go, but his fear remained.

My question is how common is it that previous fairhful partners cheats themselves instead of just the options 1. Trying to work it out 2. Leaves?

Updates
7 mo
I think my partner has placed a fear, a trait in me from his previous experiences, but then again, Im not commenting on his Ex now, but in general terms he's had his play period and he too has been played on. He said his jealousy of his past girlfriend was not existing and he could for some reason allow more, but as soon as it came to me his jealousy hit the roof at any imaginary threat, he wasn't ready for. They say he let me in more, the higher the risk, the jealousy.
How common is it that you'll cheat back?
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