I don't think Yoon will sleep with him next chapter. Not enough build up for the pace this writer sets. The sleeping together will likely be saved for season 1 finale.
Plus Yoon still has the other nobles as guests and they likely expect a Midnight Orgy Hour. It's too risky that they'd get caught. When and if Yoon does sleep with Painters, I doubt he wants the other nobles walking in on them. He has no reason to rush. It's not like the Painter can leave.
He can take his time with the kid. Yoon has shown spectacular ability to manipulate things. He'll take his time with the Painter, but if he wants him he'll have him.
This is my guess for how chapter 20 will go down. Obviously spoiler alert.
Nakyum is going to innocently seduce Seungho since he thinks Seungho is the teacher (forgot his name and don't care lol). We all thought NK and Seungho's first time would be a forceful rape like the forced jerking off in chapter 16. But because NK is drunk, I think he's actually going to be the aggressor and be really sexy because in his mind, he is trying to seduce the teacher. Obviously Seungho will be very turned on but I think he'll also be sort of moved by NK's innocence and vulnerability.
It would be a contrast of love-making versus the horny orgies he's had up to that point. Anyway, it's just a guess. Can't wait to see what happens next week!