You're judging based on that part alone and not the WHOLE LOT development of him. Sure, he is a bit like a bulldozer in his way to Jiheon's heart, but not without consent. And it is clumsy—which Jiheon even finds adorable.
Also, if you're reading that chapter again, you'll see that what he actually wanted was to stop having an agent-athlete relationship, not merely just having sex.
I wouldn't say it so far as manipulation. That's a bit too much. Him asking for sex is just a way to get rid of the athlete-agent relationship because he hates how Jiheon uses that to hide himself and the way he cares for Jaekyoung.
Let me quote some parts of the chapter:
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<“If I don’t have sex with you now, I think I’ll regret it in ten or twenty years.”
Jaekyoung was smiling, seemingly expecting Jiheon’s refusal. It was clear that he believed Jiheon would react with anger to such nonsense. And then, by using that as an excuse, he intended to force an end to their damned athlete-agent relationship.>
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(Jiheon KNEW his motives all along. He could've refused and be done with it, but he HIMSELF is willing to take it. He always wanted Jaekyoung to be the winner in anything, so he doesn't mind being the losing side.)
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<If he answered he couldn’t do that, Jaekyoung would smile and say that he knew it all along. He would then use that as an excuse to end their athlete-agent relationship and refuse participating in the Olympics. In either case, Jaekyoung had nothing to regret, at least for now.
On the contrary, if Jiheon said he would do it, Jaekyoung would have to continue the athlete-agent relationship he had deemed terrible. But if a physical relationship, including sex, was involved, it could no longer be seen as an ordinary relationship between an athlete and an agent.>
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In my opinion, sex is not Jaekyoung's main purpose here. I believe he could've done something else as long as Jiheon won't see him as just one of his athletes.
To whoever said jaekyung was a whole green forest you were wrong wrong wrong bro literally manipulated me into having sex with him like PLSS