Is this a good way to handle this situation with a cheater?

Ok so the guy I have been dating has a girlfriend. He doesn't know I know and I found out this morning and I hung out with him last night. I feel really stupid because I had a feeling something was up. He really just wanted to have sex with me I'm sure of it even though he said he didn't. (We didn't have sex. I'm a virgin he's not) we did fool around though. Anyways I'm planning on ignoring his texts and if I run into him which I know I will I'm going to confront him. I don't want to confront his girlfriend yet and I might tell her after I talk to him but I don't know if I should. What shouldi do?