Yup, he was very cute.
I wish I could see another expression on his face, like the smile he had back then.
I am not aware that a kid that age can give consent. Beside, being manipulated into it without full understanding of what is being done to him is also being raped.
Consent is something you willingly give without influences like"if i don't do what he wants me to do, we won't be together anymore". It wasn't consent at all!!!!!
He never threatened to leave him if he didn't have sex with him. There is no way the MC could have known that Sungho didn't fully consent or that Sungho thought he would leave him if they didn't have sex. So no, it wasn't rape. It's still not the MCs fault. I highly doubt the MC would force him into sex if he said he didn't want it or put up some kind of struggle. The MC is a bit of an ass but he isn't a rapist. On the other hand, Sungho is a rapist and on top of that one with a terrible reason to do so. (though any reason is a bad reason to rape.)
With a kid the same age? So you're trying to somehow claim he couldn't give consent then but the MC could? And his own thoughts on the matter that he didn't voice, weren't something said by the main character. So he manipulated himself? If that's where you're going, damn did he start raping people young! Hopefully, he himself was his first victim.

First few chap: you fking rapist!!!
Then the flash back came: awwn what happened to you babe, u were so cute!!
More flash back: come here! You need a hug, my poor Sungho~
( ̄∇ ̄")