It's something of a grey area really.
Being absolutely literal, for a woman to loose her virginity, she needs to have intercourse, whereupon her hymen is ruptured by the penis.
That said, some people regard any sexual contact beyond simple foreplay as 'loosing virginity'.
I would say, for the sake of argument, that a woman who has experienced anal intercourse, but still has her hymen intact because she has not experienced full penetrative intercourse, is a virgin.
I hope that helps.