koreans count a newborn baby as one year old so seunghee's international age is most likely 17 (or 16 assuming he hasn't reached his bday). and the legal age for koreans is 19 as well. like you said if seunghee is of age internationally it would've been a non-issue but that's not the case here unfortunately.
Honestly, it’s pretty interesting how these stories insight ppl to actually do research on the laws and that (myself included)
But I can’t help but think of those who have parents or someone that looks over their search history....
Once I was discussing with someone about the legality of insest regarding blood relation or not. They couldn’t look it up coz they were on their brother’s computer and I couldn’t help but think that that would be an awkward conversation to have

Wait, Seunghee is 18 y.o.? Then what's the problem? He's a legal adult. He's not practicing pedophilia on Seunghee at that point. At most you could say the age gap is distateful and that's personal preference. It's weird that he goes after them while they're still in high school, his motivations there definitely make him a pedophile (not literally, i know pedophilia refers to children age 13 and below, i just don't know what else to call his tastes), but the reason people want Seunghee away from him is because they feel like Seunghee is another victim. He's an adult. Right now we need to see how the blond really feels about Seunghee, this chapter implied he actually cares. If he actually cares and Seunghee is 18 y.o., why does it matter anymore? I know who Seunghee ends up with, but maybe, if the feelings are genuine, Seunghee could've been the one to lure him away from kids? I guess the argument would be Seunghee's mental age. Strange that the blond finally finds a legal adult he's into and the president tries to make him feel gross, i think i see jealousy. If he really thinks that blondie is trash, why is he not in bars? Feelings.