I think it's slightly misleading to go by a medical definition of virginity, when in most medical situations (ie, if you visit your GP) you are asked “are you sexually active?” Likewise, when donating blood, you will be asked how many sexual partners you have had regardless of the type of sexual activity experienced.
I am a lesbian woman who has only had female partners. If you're to define “virginity” in terms of penile penetration, then many lesbians like myself are “virgins” despite years of being sexually active.
Virginity is a useless concept when put into practice - given that sexuality and sexual expression vary so greatly between people. I wouldn't get too caught up in what society loads into that term.
I'd echo this. Sexuality is too diverse and fluid for a catch-all definition of virginity to have any meaning.
I'm a lesbian. I have never slept with a man and never had a penis in my vagina, but I certainly don't regard myself as a virgin. My own cherry-losing occurred the first time I went to bed with my first girlfriend. She didn't penetrate me, but I had an orgasm - and I thought at the time, and still do, that I lost my virginity.