What does he want? I'm confused about an affair.

I met a man through a friend. He's in another state. We talked and texted for a couple of months, simply as friends. At the time we were both married. One night he changed the texting into sexting. I was in shock. I diverted the convo back to innocent chatting. He apologized and everything went back to normal. Weeks later, sexted me again. That time i sexted back. My divorce was final, but he is still married. Finally we met in person. Seemed to be attracted to me but never said so. Shouldn't matter. Yes, we had sex. Just once. Wasn't planned and I've told him i deeply regret it and it cannot happen again. He said he agreed. I told him we can't even be friends anymore. He's in Ny. I'm not so it should be easy to end communication. What i don't understand is, we'll never see each other again but he calls every day, except on weekends when he's with his wife. He doesn't sext anymore, only talks as a friend. He's not happily married but why is he still calling me? It's difficult for me to ignore him but feel like i should. I want to know what drives him to maintain a long distance friendship with someone he's already gotten sex from who he'll never see again.He says the sex shouldn't have happened but always brings it up jokingly. Still calls and basically fights for my friendship. Why? What's he thinking? What does he want?