Is it possible for my friendship with a married man stay platonic this time?

You screwed up. It's never going to be the same. Cuz now all you are going to think about is what you did, and this is going to create feelings and curiosities which always lead to actions. My opinion.... you shouldn't even be talking to him or have a friendship with him. He's 21yrs older than you!!!! Yuck. Go find someone closer to your own age, and make sure he's not married.... You are not a good person to be meddling with someone else's man. Think about you, what if someone Fxcked your b/f or husband? would you feel ok with that?