See my post #62. That's the mid to late 1930's at least.
Wow. Thanks for the link.
Note how bisexual Cary Grant and Rock Hudson had bold scenes in movies showing the truth? One Rock scene had him having to wear women’s clothes and rush outside somewhere and a guy seeing him said “Him? I never would have guessed in a million years.”
And in Pillow Talk he jokes to Doris Day about effeminate signs like collecting recipes and a too close attachment to a mother could show something about a man, and enjoys fooling her by providing those traits soon after.