1) Girls who indulge in girl on girl action when surrounded by girls only. Despite being hetero, they get their jollies the gay way, rather than have no action at all. Common in students at all girls boarding schools.
2) A girl who announces she's gay when being chatted up by a bloke she finds repellent.
3) The heterosexual emo females, who believe that gayness=coolness and therefore have all been gay/are currently gay/will be gay very soon but will embrace their genuine sexuality when that is in fashion.
1) I first had sex when I was seventeen, if you don't count the two years in which I attended Roedean and was a convenient lesbian.
2) Phil decided to go home after being shot down by the fifth convenient lesbian of the night.
3) Those emo girls aren't actually gay, you know. They're just convenient lesbians. Next week they'll be into pre-op transexuals to coincide with the latest reality tv show.