I think you have two choices. Tell him and deal with the fall out of your lack of integrity, dishonesty, untrustworthiness, selfishness, shame, guilt and a world of pain you will see in your partners eyes or shut up, suck it up and never ever do it again... including never ever seeing this man again or talking with him again.
This to me is the reality you have caused yourself... hope it was worth it.
I'm sorry this is harsh, but having done both, I can say with a lot of conviction, you will forget what the sex was like ten years from now and only remember the pain you caused yourself and everyone involved, including future lovers, because that never goes away....
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