Yeah it's kind of an open ending. Basically she acknowledges that she has a "slutty body" (meaning she does indeed like being screwed by the other guy) but at the same time she still has love feelings for her boyfriend. She thinks of herself as a white shirt that's been irrimediably tainted, she likely believes it'll be impossible to turn back to normal but she loves the fact that her boyfriend is not giving up in trying to cure her "illness".
Boyfriend is of course unaware of the affair between her and the other dude.
The other dude loves the girl as well, and even though he keeps fucking her abusing the fact that she can't resist due to her weird illness (sluttiness), he realizes that the girl still has true love feelings for her boyfriend. He wants her to give up on her boyfriend but as I said earlier she does love him.
If there is going to be a third chapter it's hard to imagine that it's going to end happily. As long as her illness/sluttines is there she won't be able to refuse the other guy, and she might even end up requesting for sex from him. The boyfriend won't take any initiative because he's too kind and considered towards her either...
apparently the girl got pregnant i think the second last page they boyfriend said about an appointment with doctor last page is the girl saying though she cant even kiss him she love him its kinda confusing