This whole thing has reminded me of my past, about my relationship. I was having a boyfriend who, when he declared his affection for me, actually had a girlfriend. They broke up eventually because they already had issues, I'm not the third person I can assure you - but it is not the right thing to do. I mean, he had a girlfriend already and he still made efforts to become my boyfriend. I don't think it's right but I was a silly girl so yeah... I remember one thing that my friend told me "If it started in a none too decent way, all will not end well."
You know what? That's how it went down... because he did exactly the same thing he did to his then-girlfriend to me. What I'm trying to say is: always do good to others, because it will come back to you.