Green’s Dictionary of Slang

(gay) a current or former lover.

1960		G. Westwood A Minority 208: Affair n. [...] also used of the partner, i.e. my affair.
1972		B. Rodgers Queens’ Vernacular 18: [...] 2. (Brit gay sl) one’s current lover of uncertain duration.
1982		Maledicta VI:1+2 (Summer/Winter) 136: A male homosexual may be having an affair but only in the U.K. is he likely to call his boyfriend his affair.
2002		M. Houlbrook Sun among Cities intro. 8: Cyril L. moved to London in 1932, at the age of 20. [...] In 1934 he wrote to Morris, his affair.
2019		P. Baker Fabulosa 288/2: affaire, affaire someone with whom you have had a relationship.