If the concept of "losing your virginity" is an important part of your identity or your girlfriend's identity, then you can each define it as best suits your sexualities.

However, the concept has never sat well with me, and I have personally felt far more empowered by dismissing it entirely. Sex with any partner for me is simply a new (great!) experience, and I don't want to elevate my first ever sexual partner to a level above the rest simply because he happened to be the first.

When I first had PIV sex with my first boyfriend, I was the same person before and after. It was a big deal for both of us--if we had cared about our "virginity" statuses, the sex would have been "more important" to me and less to him, as he had had sex before.

Similarly, the first time I had sex with a woman was absolutely fantastic--but I don't regard her as having "taken" anything from me. Again, she had had plenty more experience than me, but it was still the first time she was having sex with me.

In my experience, the concept of virginity just serves to make everyone feel bad about themselves. Still a virgin? Not pretty enough to get laid. Not a virgin? Shouldn't have had sex. Had sex with a woman and not a man? Ha, you're still a virgin because that doesn't count!! It's silly and emotionally destructive for a lot of people.

If it's a concept that serves you and your sexuality well--go for it. If your girlfriend is happy with the idea that she has lost her virginity in that experience, it's great. I just find that sometimes people haven't even had the chance to consider that there's another option besides trying to pin down some sticky definition of virgin--and that option is simply dismissing it altogether.