Green’s Dictionary of Slang

(US gay) a marker of the fact that someone is homosexual.

1998		R. Scott Rebecca’s Dict. of Queer Sl. 🌐 San Francisco Accent — being gay.
2003		Queers and Allies Dict. 🌐 When Bank of America was acquired by NationsBank [...] The home office asked that no more employees with ‘San Francisco accents’ be sent [i.e. to NationsBank’s headquarters in North Carolina]. One can only assume that this is a new codephrase for being gay.
2005		Urban Dict. 🌐 San Francisco accent. The over the top manner in which an openly gay person speaks. A person who can be identified as being gay through their manner of speech is said to have a San Francisco accent.