Why does he keep asking questions about my sexual past?

Anonymous
I've been spending time with this guy for about six months and for some reason ever since I opened up to him about my past he has becomes obsessed with asking me questions about my past sexual partners. It turns into him asking me for about two hours non-stop! He is constantly asking me about the tiniest details and what sexual things I've done with those other guys and how many times and in what rooms. Then he will ask 'so what happened next?'

When I'm talking about one past sexual relationship I had with a guy and he will ask me if I still text him and if I still have his number. I keep telling him that I blocked him. The issue is that he becomes withdrawn afterwards and isn't as talkative to me. Every story I tell about me (no matter the topic) comes back to "and who was that with?" or "who were you dating at the time"

He always asks me to show him pictures of the guys who I slept with and when I show him the pictures he goes 'he looks weird' and says 'he looks like he is full of himself'

He hasn't expressed any interest in me before and isn't physically attracted to me, so why is he so interested in my sexual past?
Why does he keep asking questions about my sexual past?
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