The main issue was that he's never had serious relationships. I don't think it actually said he's cheated and stuff (could be wrong) so that in itself is not that bad. The thing as a tutor, yes that was obviously not good however it up to them to decide if they can move past that. So I think your analysis of him is honestly way too harsh. It just seems like he didn't really know what love was and didn't understand what to do when someone loved him until now. Because only having casual relationships doesn't make you a bad person even if people get hurt because they fell for him.
Because other than not telling him about him being his tutor, he hasn't actually done anything wrong in his current relationship. He hasn't cheated or anything like that. He has just had a promiscuous past.
No one should care about his sexlife and this misdirection is actually getting quite annoying in how this justifies his behavior. His casual sex life is the low hanging fruit in this argument. It's his manipulation,gaslighting, and callousness that intersect with his casual sex life that is the issue. Seriously who cares if he fucks 1-200 people, what matters is his intentions have always been in his own self interest and others are justuseful as only necessary to pass his whims or current fancy. Having a threesome with people after you've already manipulate your patient who you already know and should be a conflict of interest nto being single by purposely giving wrong advice to ensure he's available for you to pursue not as a maybe but within the impression of a serious romantic relationship is scummy at the very least and disgusting to anyone with a moral compass.
It's not what's he's done before Hanghyuk it's what he's done parallel and specifically with Hanghyuk at various times in their now 6 years of knowing each other at this point that makes him a morally awful person. If you can accept thats fine, it's exhausting having to make excuses for shitty people but people do it everyday.
Maybe I'm forgetting parts of the story but I really don't know where any of this is coming from? Idk, if you could actually say the plot points where these things happen
It's funny how many essays out here rationalizing Dr. Han because we've been conditioned to accept the bare mininum of human decency. Dr. Han is not a good person. The author can redeem however they like and there are those who will readily accept it. But given how this character has been presented, his priorities, his actions, and lastly his rational he is not a good person. The end. Why he'll be reawarded in the end despite this all could be answered by the patriarchy, male privilege, society's forgiveness of "men with bright futures". Buy why he's rewarded today is because this genre has decided his happiness is more important than of those he's harmed and affected for the sake of a happy ending narrative we continue to force for the sake of our "perfect fantasies".