Most Helpful Guy
Now that shows some really shitty morals on him. He should have told you about his girlfriend long before you went to his place. He has no excuse, he had plenty of chances to tell you.
I'm not sure what your intentions were with this guy. But even if you didn't feel bad about his gf, what makes you think he wouldn't do the same to you in the future?
Cut your losses and just ignore him from now on. There's no need for a dramatical ending.
Most Helpful Girl
If he cheated on her what's going to stop him from cheating on you? Have self respect and confidence and stop communicating with him. If you need to give him an explanation say that's not what I signed up for.