I thought this was supposed to be slightly creepy and not represent a normal and healthy environment. Harry was in that regard the innocent one. The Slytherins are in an extremely fucked up culture with often times extreme power disparities, even among the children, which causes these situations. I thought the situation with Daphne was handled especially well, because children who grow up in such an abusive environment don't act like adult rape victims and withdraw from sexual contact altogether. (I'm overly generalizing here, both characterizations don't apply to everyone in that category obviously.) I actually thought the sudden move from Daphne unto Harry was a mood switch which can happen often in such individuals. Of course I don't have a problem with young teenagers having sex (at least as long as no power disparity exists between the partners), but the portrayal in the story was obviously negative. This wasn't supposed to be normal.