Okay but I do wonder if they were "together" when he had sex?.Just being honest but a first date doesn't solidify a relationship. It means that you're feeling each other out to see if you want to commit to each other. Now if they had already had dates and it seemed they were monogamous then yeah what a douchebag. But. If they had not gone a day before and they had not said oh I want to be together then honestly the doctor was not in a committed relationship at that moment.
Spoiler
They new each athor the doctor was the ukes substitute teacher and they had a pretty good friendship and had feelings for each a aother and if not mistaking they had sex too... Doctor's appearance really changed through years he used to have a biker look's you know that badass boy vibes but I think his father forced him to separate and to study medicine... But he never forgotten the uke( the uke was badly hurt when they got separated) ... So when they met again seme knew who he was and uke didn't so he tried to get back together with him without revealing his true identity... I'm not telling what happens next
So this is what I could figure from the raws I don't know Korea tho
Yep and it makes everything worse.
He practically tricked the uke and made him fall in love again even though he didn't have honest feelings for him... For some people sex is not something deep but for me if a person is interested in someone and is trying to have a relationship with that person they won't fuck other people just because they're horny... Guys like that are rotten and don't deserve to have their feelings reciprocated.

SPOILER.
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I saw the raws and honestly I didn't like the story, especially the fact that the doctor had a sex friend and was having sex one night before having a date with the MC. Apparently he already liked the MC but still fucked this sex friend ONE night before they met... WTF (〜 ̄△ ̄)〜