There’s not much evidence, but I think it could be true. Consider that in Prick Up Your Ears, the Beatle picking up gay playwright Joe Orton to discuss another Beatle movie screenplay is McCartney.
Consider also the lyrics:
“Jet”: “And Jet, I thought the major was a lady suffragette.”
Happy ever after in the market placeOr “Get Back”:
Molly lets the children lend a hand
Desmond stays at home and does his pretty face
And in the evening she's a singer with the band
Sweet Loretta Martin thought she was a womanThey keep explaining these as mistakes. Well, Mr McCartney sings this soi disant mistake over two lines; more than a mere substitution. Also note the matching of the pronoun: Desmond does his pretty face.
But she was another man.
All the girls around her say she's got it coming
But she gets it while she can.
Perhaps Mr McCartney simply enjoys sexual confusion as a comedic gambit: he’d be hardly alone. People like casting Lennon as the adventurous one but there are good reasons against that. Lennon grew up without a father or other male role model. McCartney grew up with a musician dad and apparently learned a lot from him. The latter type tends to be more secure and more willing to experiment (or be seen as such). Lennon’s childhood would create a personality more likely to be hostile to bi- or homosexuality.
Again, no proof. And yes, I am open to charges of projection, I know. But I do wonder.