Virginity is a social construct. If you had only had sex with women all your life, then by those standards you would have always been a virgin. Which means every Gold Star Lesbian would be a virgin right now, no matter how many women they have slept with. Obviously, this simply isn't true.
When a man loses his virginity, what does he actually lose? Nothing at all. Nothing changes. And yet, they still consider it "losing his virginity" whenever he has sex for the first time. So why should the loss of a hymen be what decides a woman's virginity? And besides that, lesbians can lose their hymens from penetrative sex, too. And your hymen can break from normal activities or stay in tact after having penetrative intercourse. So no matter how you look at it, it's stupid. Simple as that. Virginity is a stupid concept and you've lost "it", whatever "it" may be, as soon as you've had sex. Period. Male, female, it doesn't matter. You've lost it.