Why did he reject me in the middle of sex?

This has been bothering me all week. This past week I got together with an old lover/sex friend/guy I went on a few dates with who now happens to have turned into a friend. We haven’t had sex for 3 years or so but we have fooled around and made out quite often since then. I’m in an open relationship and my boyfriend was out of town. He came over to my place that night and we made dinner and cocktails and were laughing, catching up and having fun. We both got pretty drunk, and later I gave him half of a space cake. I’m still really physically attracted to him (as well as secretly emotionally attracted to him). We were sitting close and I started trying to seduce him. Making eyes at him, smelling him, sitting on his lap, grinding my crotch against his. We ended up making out pretty passionately. He was pulling my hair, kissing me passionately and it felt so good. I then went down on him and gave him oral sex, grinded more on his lap and he fingered me. I was so turned on so I led him to the bedroom where I gave him more oral sex. I then sat on top of him and started having sex with him. We had sex for what seemed like the next 15 minutes maybe 20 minutes. Then all of a sudden he stopped and was like I can’t do this. I can’t come. My buzz wore off. I don’t want to make things weird between us. And I was like ok and we went to sleep. Well I couldn’t sleep so still drunk I went down on him again. He got hard again and let me suck him till I asked if he would just finish it off with me and let me come on him. He got really angry, got out of bed, put his clothes on and headed out the door, leaving half his stuff behind. The next day he texted and said it was a weird night, he had so much fun till the end. He said he regretted going into my teasing because our last fight had the same basis. He said he went into my teasing because he was horny, but that he didn’t think of me as a sex friend as it had been so long since we had sex. I just don’t understand it. I asked him why? I asked him if he didn’t think I was pretty. He didn’t answer except to say that I kissed him. But I just don’t understand what is going on. I mean if he doesn’t think that way about me why do we always end up making out every time we see each other? Why does he get turned on when I touch him? He’s told me before in the past that I was sexy. But I just feel like it must be because he doesn’t think I’m attractive anymore. But that doesn’t explain why he still kisses me and gets hard. I mean I thought when men found you attractive they couldn’t say no to sex. My ego is pretty hurt. The most vulnerable time during sex and he rejected me. Ouch. Any thoughts on what is going on with him? Why he does this to me?

He goes on vacation for a few weeks to Spain, sends me racy sexual text messages the whole time. Then he gets back and this happens. I just thought since we used to sleep together and he used to act attracted to me that we'd be on the same page.
Why did he reject me in the middle of sex?
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