Green’s Dictionary of Slang

(gay) a heterosexual person; also attrib.

1910		I.L. Pavia Jahrbuch für sexuelle Zwischenstufen No. 11 41: B.M. (Verkürzung von bloody man) ein wirklicher Mann (als Antithese zum homosexuellen Mann), auch Womanizer [GS].
1925		R. McAlmon Distinguished Air (1963) 19: The chief of police in Berlin is as queer as they make them himself. Anyway you needn’t worry, they recognize that you’re the B.M. stuff.
1932		T. Croft Cloven Hoof 65: ‘B.M.’ or ‘Hearty’: heterosexual, normal, virile.
1949		‘Swasarnt Nerf’ et al. Gay Girl’s Guide 4: Abbreviation for ‘Bloody Masculine’, BCN [British Commonwealth of Nations] term synonymous with butch.
1979		Maledicta III:2 244: Heterosexuals, outsiders, jam, normals, squares, BMs (‘baby-makers,’ Cape Town), and general naph omis.
2003		K. Cage Gayle 55/1: BM n. 1. heterosexual person (abbr. of Baby Maker) 2. heterosexual male (abbr. of Bloody Male).