Karly knows exactly what is going on with Sophie because Sophie told her.

Karly fancies Kris.
Charlie fancies Kris.
Kris fancies himself.
Kris is struggling because if he weren't on BB he would have had a one night stand with Sophie by now and moved on and is unable to follow the usual sequence of events because she is there every day and he has to face her and live with the embarrassing consequences.
I don't see that her being inexperienced has anything to do with it, as he tried to claim he wouldn't want to hurt her. Well, by ignoring her and hoping she will just disappear will hurt her also. So he doesn't care, he can't do.
He takes it as written that even if they were together he would soon be interested in somebody else, so he obviously has no feelings for her at all as if he did, that wouldn't be occurring to him. He has "conquered" her, she was an easy conquest and is already bored and that coupled with sexual frustration as he was unable to follow his usual route just makes him withdrawn to some extent.
Sophie didn't want to be "full on" she says, which is fine. It seems to me a stalemate situation and if only they would just say so to each other it could all be laughed about and moved on from.
Sophie doesn't need him and she doesn't need to feel responsible for his crappy decisions.