Grabe ang character development..

Personally I think he said that because he is slowly realizing his own feeling toward Nakyum! And for him that's strange because he probably never felt like that before

I think Seungho is a bit perplexed about Nakyum's current string of actions. In the beginning, he's frustrated that Nakyum doesn't like to be touched sexually or have sex with other men. Then Nakyum sort of does and pretends to like it (he asks Seungho to do him), so he (Seungho) is confused and also suspicious. Well, who wouldn't be? No one just does a 180 out of the blue.

Add that to the fact that no matter how many times Seungho asks what caused the change, Nakyum never actually gives a truthful reply. There was one instance where Nakyum bursts out that Seungho keeps him there for bodily pleasures, which Seungho admits to. I think he also expected more anger and retaliation from Nakyum.

Then we move on to the recent chapters, and we see that Nakyum is discovering that (1) he doesn't really have anywhere else to go, and (2) having sex with Seungho is actually not that bad. Also believing that Seungho only keeps him there for sex, I think Nakyum's thoughts are more or less, 'Sure, I guess. What do I have left to lose? This isn't so bad'. He doesn't try too hard or act whore-y either, he doesn't use seduction tricks on Seungho.

Seungho doesn't know this, ofc. So I think after admitting that he keeps Nakyum in his castle for bodily pleasures, he's expecting Nakyum to act all whore-y and seductive, OR burst out into tears and retaliate like before. However, Nakyum does neither. I think Seungho is confused about this reaction from Nakyum.

With chapter 50, we know Seungho has found out that Inhun called Nakyum a whore. He knows Inhun is trying to use him to gain more political power, and that he's using Nakyum as a pawn. Seungho probably also thinks that Nakyum isn't aware he's being used as a pawn. So for Seungho, these questions might arise:

- If Nakyum is doing as Inhun asked, why doesn't he try harder to seduce?
- Since Nakyum isn't trying, does this mean Nakyum ISN'T going to do as Inhun says, or have they had a falling out?
- Why is Inhun still such a bother to Nakyum even though he's sent him out of sight (and hopefully out of mind)?

I think Seungho is vexed because he seems to have an inkling that there is a falling out between Nakyum and Inhun, and he suspects that Nakyum no longer holds feelings for Inhun. Yet I think he also picks up on Nakyum's reactions, which are not "Oh I love you Seungho" vibes, BUT also not "No sex please" vibes. So I think his remark is hinting at his current state of mind, which is confused as to where Nakyum stands. NK isn't saying "I love you", but he also isn't saying "I don't want sex with you". NK also seems to have a falling out, but then why does what Inhun say affect him (NK) so much?

Yeah, this is how I'm reading character relationships so far.

We don't know what Seungho found out exactly.
If the servants in the library heard the whole conversation, they heard Inhun saying that he was expecting Nakyum to spy on Seungho and when he didn't he suggested that Nakyum was getting close to Seungho. After that he humiliated Nakyum with the prostitute comment.
Maybe if Seungho is listening all this including the part that Nakyum was instructed to spy on him but finding out that Nakyum didn't betray him when he got the chance is what he is confused about.
After all, despite knowing that Nakyum enjoy the sexual part of their arrangement, for all he knows Nakyum hates him and is in love with Inhun so it would really strange indeed that Nakyum disappointed his beloved teacher for Seungho's sake.