In case you don't know -- "sex" isn't just penis-in-vagina penetration. "Losing your virginity" simply means having sexual contact with another human being for the first time.
Sex is any intimate contact between 2 people that results in an orgasm. So genital to genital contact, genital to anus, mouth to genital, fingers or hands to genital....all of these are sexual acts.
If 2 girls have intimate contact with each other, its sex. And if one of those girls have never had any sexual contact with anyone else prior to this, then she has now lost her virginity.
Get it?