Would you tell me what to do?

I once had a boyfriend and he cheated on me in the course of 3 months. Since then I basically been single. I guess I’m not lucky to date someone. About to be 25 and unhappy still. The point is this guy is taken and we have kissed but nothing else. He clearly wants to fuck. But I feel if I have sex with him because he’s married maybe in the future karma will happen to me. So that’s is stopping me from doing so.

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And this is because the guy I liked had sex with someone else. While I promised I would wait for him to come back.
Would you tell me what to do?
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