I keep sleeping with a guy in a serious relationship
Why would you sleep with someone who is in a relationship with someone else?
I just want to.
Even if you don't understand your own motivations, you should stop doing something that is unethical and unkind to the other person's partner. Helping someone cheat is a bad thing to do.
I never said it is okay but I want to do it.
Even if you want to do it, you recognize that it is wrong. You need to stop sleeping with him.