
I bought 35 on Lezhin, and
**spoilers for the scrollers*
He did actually sleep with blue haired dude. Yubin wanted to save his first kiss but gave away his first time. (on a side note, that means that during the extra chapter where he and Ginu kiss in high school he wasn't a virgin ='D)
Also, Korean school years are different from ours. At 16, it was his last year in middle school, and since he's 20 now it's his first year of university.
It's completely possible that the manhwa artist retconned Yubin's cherry loss in though, since as you said, the early chapters implied he's a virgin.

Oh God!!! I just can't believe this! It's so frustrating when the authors retconning like this!!! I'm an author myself and it's very important to stay stanch on your original plot, because your former information can't lack in addition with the later statement of your story and about all the small details around your main characters. There are a lot of things that don't make sense when you do this. Words and actions must follow the same direction!
( ̄へ ̄)
If you didn't read the raws don't read this!!!
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In ch. 8 page 2, Yubin says that he only know sex from having fantasies featuring Ginu. In the same chapter, page 8, Ginu make a research on the internet about how you can be good at sex.
Also, in ch. 11, we see Yubin buy a dildo, after he enter in the university (when he was 18, I suppose), and he says that he didn't have the skills or the knowledge of how he supposed to do it right. Besides that, at the present time, he makes a lots of thoughts about how much is the difference between doing it alone and having a real sex partner.
I'm a little bit confused about all that theme with the silver haired guy, his brother's friend. Yeap, they have met each other when Yubin was 16 (2 years before he bought the dildo I mention earlier), yeap, the went to a hotel together, after the incident at the gay bar, yeap, they were about to have sex, but... we don't actually see them doing it. And when Ginu and Yubin talking about this, it's not very clear what really happened back them. It looks like that Yubin talks about this a little bit unclear, on purpose, because he wants to provoke -and hurt a little- Ginu, who had slept with lots of girls in the past, at least inside Yubin's mind.
On the other hand, Ginu maybe had kissed some girls, but he never had sex with any of them. Yubin is really his first and he says that again and again (even after they have a fight in chapter 35, when Yubin FINALLY confess his feelings for Ginu).
I'm very confused, really... I hate it when the things aren't clear enough and when the events don't actually fit (past events, thoughts and informations are opposite enough to the fact that Yubin really had sex with any other besides Ginu... and his dildo... :p )
And I just re-read (for 257895th time) the whole story, to see if I was remember all the small details that don't make sense now.
And I'm pretty sure that some of you are going to think that I'm not very well on my minds and I can tell you that... oh, fuck it, you are right!!! ╮( ̄▽ ̄)╭