Green’s Dictionary of Slang

(US) a prostitute’s term for a client who prefers to give oral sex rather than have sexual intercourse.

1964		‘R. Blake’ Amer. Dict. Sexual Terms 16: bennie (Sl) n. A male cunnilinguist; a man who is known to prefer performing oral intercourse on a female.
1972		R.A. Wilson Playboy’s Book of Forbidden Words 34: Bennie. Whore’s slang for a customer who prefers cunnilingus to coitus.