The thing is it was always the case, because they were caught in the hospital once, Her grandma got the wrong idea. It helped her to hide her being a rental whore though, so she was lying to her grandma ever since.
During one of the arcs with Ruka, when Ruka accuses her of wanting to be his real girlfriend and proposes to just release her from Kazuya she alludes to the fact that it's more complicated then that. That's what she was talking about there.
In short : she is using him as much as he uses her, but she doesn't need to pay for it.