Green’s Dictionary of Slang

1. (Aus./US gay) anal intercourse.

1964		Homosexuality & Citizenship in Florida 26: Glossary of Homosexual Terms [...] 71Intercourse by anus.
1972		(ref. to late 1960s)B. Rodgers Queens’ Vernacular 89: anal intercourse [...] seventy-one (late ’60s, an advanced sixty-nining).

2. (Aus.) mutual oral-genital intercourse plus styimulation of the partner’s anus.

1999

R.G. Barrett Wind & Monkey (2013) [ebook] ‘Why not make it a seventy-one, while we’re at it.’ ‘What’s a seventy-one?’ ‘A sixty-niner with two fingers jammed in your date’.