Green’s Dictionary of Slang

numbers game n.

[the combination of sixty-nine n. (1) and sixty-six under sixty n.]

(US gay) an uninhibited and extended session of sexual activity.

[US]Guild Dict. Homosexual Terms 32: numbers game (n.): Combinations of anal and oral intercourse, represented by sixty-six and sixty-nine.
[US]B. Rodgers Queens’ Vernacular 142: numbers game uninhibited sex session.