It's not the harm but the influence. Dai meant that Haruki and Yusuke influences each other in a bad way, by bad way he meant that their band atmosphere is becoming uncomfortable due to their affair. Haruki makes Yusuke uncomfortable and Yusuke makes Haruki uneasy.
I'm not too sure if Dai knows their fuxk buddies, so assuming he only sees the surface of their relationship that's why he said that.
I mean get what you are trying to say but at the end of yuusuke is much younger than her and he's also not calling her out for messing with him (even tho she is older) in the first place.
If Dai ever finds out they have sexual relationship I want him to punch her. Legit I just want that bitch out.
Why the hell did he say "they are not good for each other" like what!??! That girl is a groomer and abusive how is yuusuke harming HER?!?!?