I think it’s a self value issue. He knows that’s what they want and believes they don’t actually care about him as a person. This is just him proving it’s true, combined with thinking the only thing that’s of worth of himself is his body in a sexual context. After all, they loved his body even if they didn’t love his soul.
Who is he trying to prove it to? Himself? It's already obvious that they don't love him at all but why would he need to prove it? It makes no sense at all .
Sometimes people cope by doing things that hurt themselves. It’s like how some girls think that their self worth is being pretty, and they hate that they’re being valued for their looks. Yet they’ll make themselves look as pretty as possible and accept guys that are only interested in their looks.
It's sad that those girls feel that way. Doesn't make their actions any less stupid though. Same with Jihyuk. Like why is he surprised that Yoojun is being rough with him now and ignoring his feelings during sex? It's like he forgot the last (?) night he had with him where Yoojun brutally raped and didn't stop even after he passed out.
Why do they suggest sex when they know that's exactly what the rotten semes want from them? It's so unbelievably stupid