Why does he(22M) act distant after sex? I(21F) am confused about where we stand.

I’ve been seeing this guy for a while now. I'm not sure what to label us, other than friends with benefits. However, he seems to get upset at the thought of me being with someone else, which feels unfair since he hasn't really taken any steps to progress our relationship beyond the physical side of things. I also found out that he's been sleeping with other women behind my back while I've been having raw unprotected sex with this man. Before and when we're intimate, he's incredibly passionate-playful, talkative, and really excited to have me there. But after we finish, he completely shuts down. He becomes distant, quieter, and much more careful with his words. Very intentional. The worst part is that he won't even hold me afterward. He just lays there quietly, twiddling his thumbs, and it honestly makes me feel terrible. I'm left wondering: is it post nut clarity? Does he not want me in the same way I want him? Or is there something more going on here that I'm missing? I'm really confused about what to make of all this.

TL;DR: I (21F) have been seeing a guy (22M) as friends with benefits. He gets upset if I talk about being with someone else but doesn’t seem to want anything more than physical intimacy. After we have sex, he becomes distant, quiet, and won’t even hold me, which makes me feel awful. Is this post-nut clarity, or does he not want me in the same way? I’m confused about where we stand.