The first time that Kuro calls Aoki a "virgin" is in Book 1, period 6, after he brings up the idea of involving their parents. Of course Kuro is mainly trying to protect Rin's privacy. The word she uses is "Doutei" 童貞 which is specifically a male virgin.
Manga Fox: Kodomo no Jikan Vol.01 Ch.006: Period 6 Online Manga Scans
As I recall she has called him a fool-idiot (baka, ばか) pervert (ecchi, えっち) pedophile, (lolicon = rorikon, ロリコン). Japanese makes a lot of gender distinctions, and there is a different word for a female virgin, shojo 処女.
As Nitrogen pointed out (WHERE IS HE?) even the word for "I" varies depending on whether you are male or female.