A girl's virginity
sexual acts. This is regardless of her "straight" virginity--that is, whether or not she's had sex with a man.
Since it's hard to define a physical loss of virginity between two females, lesbiginity is mostly determined by personal opinions or feelings.
lesbian: I have no real interest in men, so I've never bothered to lose my virginity. I'm dying to lose my lesbiginity, though, if only I could find the right girl!
Bisexual girl: I lost my virginity to Ryan when I was ninteen, but I lost my lesbiginity to Megan when I was sixteen!