The story
From allo- (“other”) + sexual. "Directed at or involving another rather than one's self" and "involving the other sex; heterosexual" are the most etymologically literal senses (regarding the latter sense, compare the structure of heterosexual), structural and semantic opposites of autosexual and isosexual. The sense "non-heterosexual" was influenced by (and had influence on) Canadian French allosexuel.
Etymology from Wiktionary, CC BY-SA 4.0.