Because she's into him, in case you haven't figured it out. She could be nice to him (and has been occasionally), but she doesn't want to because that's not exciting. She was lowkey pissed when he said he would only do it with "someone he liked" so she threw some shade at his non-relationship with Yui by saying he'll be a perma-virgin. Her offer to sleep with him could have been a tease, but maybe not. She definitely derives sexual satisfaction from him, so filming him would have given her some material to work with.