Hold up that actually makes sense!! He could have ptsd of being raped and it caused him to feel weak and ashamed and with men being raped it's very different from women. the way he wants to get back and avenge himself by having sex before a match to feel empowered enough to win and remind himself that he's not weak like he used to be and that is his own way of dealing with his trauma which isn't acceptable ofc but it makes you understand his mindset and why he's the why he is now (aggressive, extremely protective of himself, doesn't wanna show weakness, hates affection)
shooot i could believe this rn O_o great theory and im kinda leaning on this being the reason for it all. imma come back to this if ever a chapter reveals this ^_^;
Jumin might have SA'd him or his father did.
Jumin might have done it repeatedly whenever Jaekyung lost to him which might have gotten the 'jinx' name. Or maybe he was groomed by his father one night after he came back from gambling