Okay, but factually it's not pedophilia. It's ephebophilia. Pedophilia is classified as a mental disorder. Pedophiles can't help what they do, they'll keep doing it. Ephebophilia is just a sexual preference, it can be changed. The blond doesn't show a lack of control, he rejects Seunghee's offer to spend time together because he sympathizes with him and wants him to make friends. He isn't sick, his emotional ties can overcome his sexual preference.
So how do you explain the dick pic to Seungtaek when he was in middle school. That's sexual harassment. Seungtaek didn't ask for or enjoy seeing that as a response to his request for advice. That seems like a lack of control to me. Honestly, I thought their relationship was wrong before but not this bad, mostly because I had forgotten about other key scenes, like him trying to hook up with Seungtaak in a case of mistaken identity. Clearly the author also thinks its wrong. Regardless, of the legal distinctions between pedophilia and ephebophilia both are generally seen as socially unacceptable. I don't know where this idea that one relationship with one minor is enough to change this guy's preferences forever is coming from. Also, pedophiles can refrain from acting on their thoughts (they'd probably need help though, cognitive behavioral therapy, relapse-prevention therapy etc.).
You either aren't reading what i'm saying or just responding however you want regardless. I said ephebophilia is a preference, as in his attraction to minors. You had asked "how do you explain the dick pic to Seungtaek in middle school" after i had pointed out that he isn't a pedophile and what he's doing isn't pedophilia, this implies that you're saying him sending that dick pic was pedophilia. To this i answered "you explained it yourself" as you had just said "that's sexual harassment". Him sending the dick pic was sexual harassment, not pedophilia. I also wouldn't say that sending nudes is indicative of a lack of self control, people do all the time, they were just unwanted in this instance.
If we're gonna go by your research, then he may or may have been already past 13 years old but what if he was in year 1? He may just be turning 14, wouldn't that still count? And if it may not count, why are you so keen on making it a point that he's not a pedophile? Pedophile or not, he did a lot of things that's unacceptable.
To answer the 2nd half of your question, because it's annoying when people throw around words incorrectly. He is a "predator", not a "pedophile". All pedophiles are predators, not all predators are pedophiles. It annoys me in the same way it annoys me when people try to say that everything is rape. Rape and pedophilia are two horrible, but SPECIFIC things that don't need watered down by people just... Including things in them. Preying on children age 13- is worse than propositioning young adults with brains developed enough to reject you. Both are bad, one is worse. Grabbing a boob isn't rape, forced penetration is rape, can you see how false comparisons might be annoying and insensitive?
Your point of clarifying the terms is very much appreciated. To conclude, he is a predator according to what you wrote which is more accurate. A predator who preys on teens and sexually harasses them. Not a pedo, nor a rapist. The degree of crime might not be similar but it does not make him less of a criminal and a pervert. If anything, it just makes him a more disgusting and awful character.
Although I never wanted Seunghee to get together with him (he's too old). I was rooting for it to be a misunderstanding. It would have been nice for Seunghee to have a positive "it gets better" role model and confidante.
Also, as for the pedophilia argument, in my books it definitely is. For all the Americans out there, sending dick picks to minors will get you arrested. Grown men who prey on lonely vulnerable teenagers (regardless of how lonely they themselves are) are doing a terrible thing. Even if they act nice while grooming them.
Honestly, I think the boss is also in a legal/ moral predicament he shouldn't be allowing his friend to keep up this behavior. Asking the kids to stay away from him is not enough. He should take that password to the police. If not, get his friend some psychiatric help fast, before someone else calls the cops on him.