Is it wrong that I enjoyed a sexual relationship with a guy who had a girlfriend?

When I first met this guy he asked me to be his girlfriend. We talked and I agreed. But basically his actions didn't reflect his words. At some point I realized we were only having a sexual relationship. In the process I found out he had a girlfriend. But I still continued our sexual relationship because I'd gotten comfortable with him after almost a year of us seeing each other... albeit mainly for sex. And the sex was good. I never told him I knew about his girlfriend. I let him tell his lies.

What I didn't get was why he kept on telling me he wanted to be exclusive and serious. I was direct with him on several occasions in that I told him I knew he really just wanted sex with me. Yet he'd deny. He would also get upset when I would distance myself from him sometimes. I did finally end things with him for good which seemed to piss him off. But I also told him I knew of the other. He left me a long hostile message saying it was not true and that he wanted to talk to me about it.

Am I wrong for going along with him so I could get more sex? I figured he'd lie too so why not get something in return. Guys lie for sex so why not I.

Why would he get so angry about me ending things? Upset that his sex situation with me ended? Why didn't he just be truthful.

Mind you I said nothing to the girlfriend. She's in love with him, plans to start a family with him, and feels he can do no wrong. I know this threw a friend of a friend that is associated with her. I do feel a tinge of remorse. But like I said he lied first. Yet, my naughty side wants to start back up with him.
Updates
+1 y
I'm actually thinking about him now. Another turn on for me was the unprotected sex. Yes he's probably doing it with both of us. He's just so yummy to me. And he's an aggressive type. And controlling but somehow he has become a kinky naughty fantasy to me too. Ooh if only his girlfriend knew.
Is it wrong that I enjoyed a sexual relationship with a guy who had a girlfriend?
Post Opinion