Did you not read the last chapter when they were in the garage? Rosa said she knew about how he behaved in the clubs his father owns. Implying grape and drugs. She even said that no normal girl would date someone like him.
It would suck if he really is a black flag. After all we only saw a little tiny piece of who he was and not everything, but maybe Rosa only assumed this? (just because he thought she was crazy doesn't mean he wasn't)
Clubs are not strangers to drugged grape. It's so bad that nail poilsh that can detech drugs has been invented, of course it's probably useless now. That's what most people think of when they think of night life clubs. Go back and read through the comments. Most readers assumed this because of the way it was implied by Rosa. I also felt this possibility. (We've already seen Rosa grape Yeonwo while he was drugged and Yeonwo was forced into the life that he's in so I would not be surprised if the story made everyone bad)
I won't 100% assume that of 2ML since we haven't seen his side. He did call her crazy so maybe he did just take girls for one night stands.(I've read stories were sane people were called crazy by actually crazy people though I wouldn't stand by that as evidence of innocence)
Rosa IS crazy and she IS scary! I'm not going to defend her.

Bruh lowkey don’t care abt anyone else but Jae-on he’s the only thing that keeps me reading this so HE BETTER STAY IN THE FUCKING STORY