You know, I had already seen the raws, but to actually read the translation on that sex sc...

AGREEED!!!!

I understand why you would be upset with Jaeha, but I do believe that having sex with Eunho is just as much a desperate attempt to answer his own thoughts that he can't seem to satisfy. He is well aware that he has not been able to get Eunha's feelings and confession out of his head for 3 years, and he can't stop thinking about him since they've reunited. Sometimes he'd express frustration because of that due to the fact that he just kept remembering young Eunha so it immediately made it weird for him mentally, but we know he can get around it (which I think was the point of not having him push back against Eunha when he kissed him suddenly in the studio).
I think the sex thing is just as much meant to be a confirmation/test for Jaeha as it is for Eunho, if not more-so (even though he's the one pushing the buttons [cuz he rlly does want him he just won't admit it lol])

But there's something that has been bothering me about chapter 14 in particular, and it was how the mangaka shifted the dynamic we're used to seeing between Jaeha and his friend. I have two theories!
So, almost the whole time his friend acts half-interested and quiet. Jaeha tells him about the dreams he's been having and then calls him out for acting strangely "What's wrong with you? You've been too serious". When they start off, his friend tells him to face down and then has the gall to put the phone on speaker (what an asshat) and pins Jaeha down even though Jaeha is clearly showing his discomfort with the situation. Afterwards, before Jaeha even really catches a breath, the friend is all like nope, get up, we're not done yet.
Anyway basically what we're probably supposed to assume (and are right in doing so) is that this friend actually still has intense emotional feelings for Jaeha and is now feeling like a competitor against Eunha and thus behaves possessively to Jaeha's body that night.
The other (and much less likely but hey, it could happen) theory is that the ex-wife and him actually have a connection that is going to come around in some way/shape/form. Heck it could be both if it's twisted just right.
There are a couple other reasons from different chapters as to why I think he could possibly have something to do with the ex-wife but my bets are on the former.

Aaaaand I totally strayed off and babbled; I'm sorry!! Ch. 15 is here!

Hey Fallust, I actually agree with your analysis. I have inferred from the very beginning that Eunho's innocent and consistent adoration actually made a very deep impression on Jaeha since they were children, and I even suspected that Jaeha reciprocated subconsciously, but that he built all those walls because he felt conflicted about his homosexuality and/or his guilt for actually feeling "things" for Eunho. I think that's the reason why he got married, as a sort of defense mechanism to fight against himself. I mean, no wonder his marriage failed, because he really didn't love his wife, at least not in a sexual romantic way, and the angst over his failed marriage is not so much that he misses her, but that he feels like a failure for not being able to suppress his "base" instincts; and that's the symbolism of wearing the ring, it serves as his limiter, as the thing that stops him from coming out completely and giving up on conforming to society's expectations; and when Eunho comes back into his life, it makes him even more anxious because he re-ignites all those guilty feelings and forces Jaeha to face himself, to be honest with himself, which he hasn't done yet :/ and that is one of the reasons he is so harsh with Eunho.

This is why I can't stand him, because he's a coward that refuses to face his issues head on and instead escapes his reality in the comfort of the sexual relationship with his high school friend while stringing Eunho along because he does in fact care for him and actually enjoys basking in his adoration, despite knowing he's hurting Eunho with his constant half-assed rejections. Now, what is interesting here is that Jaeha may not consciously realize yet that he has had feelings for Eunho for a very long time now, probably since they were both children, and his sexual awareness of Eunho was probably kindled three years ago when high school Eunho confessed to him (which if I'm not mistaken he married not long after that), but even if he himself didn't realize this, I'm almost positive that Jaeha's lover does in fact perceive that there is something special about Eunho, just look at his reaction here and on the next page when Jaeha "casually" tells him that he encountered Eunho again:
http://www.mangago.zone/read-manga/lover_boy_lezhin/uu/lover_boy_lezhin-chapter-5/18/

I thought his friend's reaction was atypical; why the exclamation marks? why the emphasis on his surprise that Jaeha and Eunho met again? why should he be shaken to that extent over two childhood friends re-acquainting themselves? I interpreted his marked reaction to mean that he "knows" or perceives or has always known that Eunho is someone of significant importance to Jaeha, there's no other explanation for that, and from that moment he feels that his relationship with Jaeha is threatened and starts acting possessively, like going to Jaeha's studio to have sex with him, even though as Jaeha says, he never liked visiting the studio before. So yeah, I'm confident he's in love with Jaeha and has been since high school and he himself felt confident of their relationship until Eunho comes into the picture again, then everything changes. He's no longer willing to give Jaeha space to reconcile with his feelings, he's now possessive and forceful; obviously he knows Jaeha better than Jaeha knows himself. Which again, makes me detest Jaeha even more, especially after these last episodes, due that I agree that this "test" is more for his own benefit than for Eunho, because he's the only one who hasn't been able to define his own feelings, but again, he's not thinking about the potential devastating consequences for Eunho, he's being utterly incontrovertibly self-centered and thoughtless.

That was a magnificent response, and I totally understand!

I hadn't even considered that perspective on the relationship Jaeha had with his ex-wife...wow. I thought he really did love her (even romantically), but that perspective makes soo much sense it just blew my mind! Seriously, wow...what the heck.
You brought to light a couple of things about Jaeha and his lover that I didn't also notice before (or at least didn't think about when forming my analysis). Now I can totally see that; for some reason, I was under the impression he had dated this lover before, but it must have only been a fling back then as well. Interesting!
This all makes me have even more questions! I wonder if we'll ever find out why they really divorced, and what's going to happen now that they've reached a point of no return!