The brother is of age it was said in a chapter and Jaehwan waited for that time to actual to have sex with Byul's brother. At least he waited and has a conscience.
I feel you. Despite knowing he's a consenting adult he's drawn to appear more youthful and his mannerisms imply innocence that is more becoming of someone going through puberty and having unexplained urges than that of a 20 year old. Contrarily, J obviously has experience and gives off this feeling of talking advantage. He knows Dongho's only frame of reference is BL comics and still pushes him to his naughty limits. I'll concur it does give off creepy vibes as doing something that's legal does not mean he's doing something that's right, but all signs point to Dongho being into it and even seeking J out on occasion. I don't think it's meant to look like he's pressuring him but rather like Dongho is just easily embarrassed. To each, their own.
I really don't like the second couple. I liked the main couple because of their chemistry and their very healthy dynamic. The second couple is the opposite of that and even though I know the little brother is like 20 or something, the other guy just gives me predator-vibes. Also both characters are varying degrees of unlikable for me. Still reading this for the main couple tho. (If you enjoy the second couple, that's great, but I just don't like them)