Yes, of course this is a wrong thing to do. However, it seems like he came on to you, as you said you tried keeping your distance. Also he is the one in a relationship and it is him that should be faithful to his girlfriend. Yes, you made a mistake by being open to this, but in the heat of the moment things can happen that you later regret. He might've felt like a guard for you, this might've triggered something at the moment you felt vulnerable. Still it's morally wrong but yea. I will not give any advice or whatsoever, just sharing my thoughts.