safety date

A girl who'll go out with a gay guy who's not out to anyone else, because she knows he's safe and because he can use her to maintain his "straight" persona.

Also:

A guy who'll go out with a lesbian girl who's not out to anyone else, because she knows he's safe and because she can use him to maintain her "straight" persona.

"Hey, take a look, Kyle brought a girl to the dance! I thought he was gay."
"He's gay, she's his safety date."
"Cool."