Is it normal he got back with his ex?

I've known my friend J. since the first week of university. When I met him, he was going out with a girl from home (Fran) and I also had a boyfriend at the time, but he was still very flirty with me. Him and Fran split, me and J. kept hanging out alot, and then one night he outright asked me if we were ever going to have sex. I was annoyed and contemptuous that he'd been pretending to be my friend in the hope of getting some, and cut off all contact. A few months later, he came up to me and sincerely and profusely apologised, I was cold to him but after a while we became friends again. To cut a long story short, one night a few months after that he walked me home when I was blackout drunk, and had sex with me (I don't remember any of this). I was really annoyed that he'd taken advantage of me and again broke off our friendship.

At the start of this year, over 2 years after I met him, he again apologised to me, and seemed such a changed person that I accepted. He's not the sort of person who is comfortable apologising, so I took these efforts as evidence of the sincerity of his feelings. I welcomed him back into my life and he really has changed alot. He is incredibly kind and considerate of me, basically acting as a surrogate boyfriend. Eventually, we started sleeping together as a kind of friends with benefits situation - I started to get slightly concerned that he was becoming too possessive of me, so I slightly pulled things back a bit and didn't see him for a couple of weeks.

He comes round to my house. We were going together to a fancy dress party, and he started trying to get me to take my clothes off in front of him, and try my costume on for him, which I laughingly refused, but I showed him it on the hanger and he told me he couldnt wait to see it on me etc etc...This somehow led onto a conversation where I asked him if he lost his virginity to his ex (Frannie). When I mentioned her name he started laughing and I asked him what was up. He said he'd got back together with her during the time when I hadn't seen him.

At this point, I was really like, what the hell? I know full well that he would like to keep sleeping with me (although he doesnt want to cheat) and he'd liked me since he first met me almost three years ago...so why did he suddenly get back with his ex? IIN?