When they have sex.
Whatever form of sex they choose, oral, fingering, scissoring, any of it, is when they lose their “virginity”. Heterosexual sex is defined by penetration, but that’s not all there is to sex! So, for lesbians, it’s what they personally define as “sex”. Not all lesbians agree on what crosses the line from foreplay to sex, it’s a subjective circumstance. So it’s different for each couple.
Sumptuous Daydreams 