I can think of four possible candidates. What say you?
First contact?
Penetration?
Her orgasm?
His ejaculation?
I think penetration is the generally accepted threshold. Certainly not the last two, which may never occur.
When you break the seal and the little button on the lid pops up.
Well, virginity is a nebulous, troublesome term, but I think “penetration” is, by far, the most commonly understood answer.
Not till I’m through with em! 
When she decides it has.
When the penis has entered the vagina.
Me? I’m a guy.
She might dicker with you on that point though…
Virginity ends with the penetration. It resumes when she breaks up with the guy.
But what if she’s a nincompoop?
I’m not simply being a smartass. I knew a girl, freshman year in college, who was unknowingly pregnant first semester, had a baby over Christmas break, and came back the next semester insisting she was still a virgin because her one episode of two-backed-beast-making had not been sanctified by Jesus and thus didn’t count.
So a lass who’s only ever lain with other lassies, but has had many such partners and much orgasmic, unphallused sex, remains a virgin.
Hmmm…yeah?
I truly don’t know when a lesbian who has not slept with a man loses her virginity.
When the dildo penetrates the hymen.
What about a girl dildo’ing herself?
Is it the act of having someone else insert the dildo that takes the virginity?
This obviously can’t be the criteria. Otherwise a woman could have sex 100 times but still claim to be a virgin if she never reached orgasm.
And if she never acts as the receptive partner?
<shrug> I got nothin. If a sexually active lady never gets penetrated. I’m not sure virgin still applies. But I have no idea what term would.
What if the only penetration is non-consensual?
That’s quite the contrivance. As far as I can tell, she just morally exempted all sex from sin. I wonder how far she could take this…
“Judge, the murder doesn’t count because it wasn’t approved by Jesus!”