I think she did that so the others wouldn’t suspect her cause she wasn’t doing anything to cassis and that would’ve been suspicious so yea she made it seem like she was using him as a sex toy.
Basically she wasn't hurting him or anything which wouldve been extremely suspicious since he's her "toy" that she should "break", so, she decided to give him hickey to mark him as hers like property or something and implying that she's "using him"
Hickeys*
So the family believes he is her play thing
Early spoiler question:
Can someone remind me again why in chapter 13 she gives him a bunch of bite marks and hickeys?