Ok so their are 2 ways i can spin this the obvious way and the fked up way. (These are just opinions). Btw if anyone has spoilers pls tell so I could be wrong about the second one lmao.
The obvious way would be the fact she could be in a abusive relationship that she cant get out of and even if she wants to the boyfriend has blackmail material on her.
The fked up way would be that she coming from a family that has a lot of expectations placed upon them she feels "stressed" and wants to to deal with that stress in "unorthodox" ways such as her being tied or bound or "being slapped around" this is more like psychological approach but what do I know this is just an opinion as stated above.
BF-kun can’t be too bright. He’s attempting to control and abuse the daughter of a billionaire. He may as well just smear honey on his dick and lock himself in a cage with a Ratel, since that would preferable to what Arata or the Father might do to him.