Once we kissed and I found out he has a girl friend. He ignored me for a while, then we started to talk again. And yesterday we met after a long time and we had sex and now he is ignoring me. Is there possibly something wrong, or did I do something wrong?

I feel the person you're talking about isn't honest with you or to himself. He seems to be getting involved with you just to satisfy his urge. He doesn't seem to respect you or your feelings. As a person it's always better to be with someone who is honest and who would stick around even when things aren't working out. He wasn't honest to his girlfriend when he kissed you and probably won't be truthful to any girl he dates in the future as well. People like this rarely learn until and unless something shameful happens for their wrong behavior.

He kissed you while he had a girlfriend, and then ignored you. Then he has sex with you, and followed by ignoring you again. So it is apparent that he has a pattern of using you when he wants something from you, and then ignoring you when he no longer cares enough to contact you, after he has gotten what he wants. He has no interest in you other than as a sexual object. You're not doing anything wrong, but there probably will never be anything meaningful between you both - firstly because he does not seem to have an appreciation for commitment (kissing other girls who are not his GF) and furthermore because he has no respect for you at all (ignoring you after having sex with you, ignoring you every other time also says this very clearly). You need to decide if you're OK with him using you like this.