So she's not sleeping with him as most people thought,but IF she was,then I would assume that it's an extremely toxic relationship where she trades sex with him for him to take care of her mental and physical needs during such breakdowns.
But honestly, even if its not something like this my man still needs to get out. Realistically, nothing good comes out of being involved with such a mentally unstable girl but since this is a manga he is probably gonna stay and by end of this arc she will be all right.