She knows Chiaki is a girl though as Chiaki was wearing a female uniform when they were in school together. I mean it's unlikely a guy is going to wear a skirt at school.
@talltales read the chapter14 TL's note as well as the comments, which pretty much explained it there. Basically I think Reika acknowledges Chiaki as male, not female.
So who was that Reika supposed to fuck with? Some random dude or childhood friend? I'm not surprised with her fooling around but I'm genuinely surprised that she doesn't mind to be fucked in that cramped apartment considering she dresses like a glamor model