@Nehona
Actually, in xianxia, it's rarely to show how despicable the aggressor is. It's considered normal in their worlds. Women are 'treasures' to be claimed, killed for, abducted, raped. They're also always weaker than some guy of their own cultivation level, ergo they can't protect themselves in a world where might makes right, so they always wind up raped unless they live a sheltered life as a princess or young mistress of a powerful clan. Though both could be sold off into wedding arrangements they don't want which will also involve them being raped. Not having a dick in a xianxia story is pretty much the worst thing you could do. It's not too uncommon for wuxia/xianxia MC themselves to also be rapists. It's one of the many tropes of cultivation novels based on the backwards thinking of Chinese society. Women are weak, men are strong, and gays are creepy perverts who have to be killed (and are almost always depraved villains - which is ironic, the men are so afraid of being raped when they're rapists themselves, but the stories are written to make it seem like being raped by a gay is the worst thing ever, and raping a woman is just a bit of fun). Also, bisexuals don't exist and lesbians are a myth.
As for why she did it, I can understand it to some degree. She wanted to consummate the love they share before she abandons it to follow her path. He wouldn't have agreed and instead wasted the night trying to get her to stay instead, because he's an actual good-guy MC. Maybe she wanted to get pregnant, maybe she just wanted to have this carnal knowledge of him before never getting to see him again. I can kinda see the thought process behind it.
Fucked up? Certainly. Makes SOME kind of sense, yeah. But, I'm not going to fuss over it simply because it's likely not rape in his mind. He's upset that she's gone, not that she date-raped him. I can't see him as a victim (since he doesn't see himself as one), so I have a hard time thinking of it as actual rape.