
Well, Youngwon's fucked in the head after seeing Hohyun die over and over again and with himself dying as well. Not to excuse his actions or anything, since I also don't like the random smut scenes during a fucking zombie apocalypse. But he's definetly developed an obsession with Hohyun and so his actions are more abnormal now, since Youngwon would never have behave like this before the apocalypse. And knowing how nice Hohyun is to a fault, he kinda lets Youngwon act clingy.

He was a pervert way before the apocalypse (if you've read the backstory chapter for ML you'd know he had designs on MC since the beginning) its just that the constant groundhogs day situation made him crazy and basically given up on life other than the MC. He literally at this point does not care about anything other than hanging the MC remembering him and living happily with him.
ML is acting gross, he feels like a creep. then the author twists it around and wants us to believe MC outta the blue might not be able to control himself? gtfoh