I have a theory that it was his body responding to a new stimulation that he has never felt before and feeling overwhelmed, I'm autistic and when a sensation overstimulates me I respond very physically, for example feeling nauseous or crying. To what we understand he is practically a blank canvas, all the things he goes through are new to him, and taking in account that by their first kiss Guwon wasn't there emotionally, it probably overwhelmed him the feeling of someone else over his face. When he becomes invested emotionally is that he no longer has qualms about the physicality.
The only thing that can be deceitful is his line in front of the mirror after throwing up, when he praises himself, that could be one of two, a little clue that will makes sense if my theory is proven wrong and he is actually also deceiving the Teacher, but idk, I kinda don't buy into it. Or the second one, a red herring written by the author to put our attention on Guwon, this one can be explained on we can tell Guwon has a high self image of himself, he is very self confident, so I will take it probably as a little quirk of him that when something makes him uncomfortable he compensates by praising himself of being mister Gigachad (lol).
I heard one person make a theory that he actually has the disease that is turning everyone into zombies and somehow was saved? Which explains his eyes being slightly bloodshot and white pupils. And the black ink on his hands is hiding the discoloration from the change. And he puked because his human self is fighting the zombie self left in him that wanted to eat the others. But he is trying to trick everyone. But idk. Theres too much up in the air atm ┗( T﹏T )┛
Well, you know... Both your theory and the other pretty souls theory are quite nice.
For me, ml is hiding something, Guwon is highly praised by both non violent and violent people and that makes you wonder why... Because the violent people have access to weapons, so they should be able to "kill" Guwon whenever they want.
What does Guwon has that makes others not kill him and follow him unconditionally?
Why is the male lead so resilient to this situation? It's not like his reaction is normal...
I want to know more O.O

Im still really concerned about guwon.. I remember him throwing up the first time they kissed. Still makes me nervous (⊙…⊙ )