I dont think I've ever heard the take that girl seems to be about Paul, I mean, it makes sense absolutely, but can you expand some more?
Rob Sheffield (Dreaming the Beatles) said he thinks Girl is about Paul in this episode of @anotherkindofmindpod. The episode is actually an in-depth discussion of In My Life, but Girl came up a number of times, since it's also on Rubber Soul.
I thought Sheffield's statement was interesting, and not in a silly “John saw Paul as a girl” kind of way.
Summarizing mercilessly, and taking a few steps back before returning to Girl:
RS argues that Rubber Soul marks a moment when the Beatles’ songwriting moved from a commercial/craft perspective towards a more open/confessional/personal tone, In My Life being an example of this, with John examining his feelings for all his friends and lovers, and singling out a new kind of love that transcends the loves he’s known before. According to RS and the hosts, In My Life is not only addressed to Paul (I personally feel it could also be about Julian, or about both; as someone who writes, I really feel the “a piece of art is never about just one thing” argument)— it also, by summoning a group of dear people and openly expressing his feelings for them, emulates Paul, who, in John’s eyes, is the more extrovert and socially comfortable of the two. The song is a two-fold tribute.
Girl, still according to RS, forms a matched pair with In My Life, because it, too, concerns complex and intimate emotions; in this case being unsettled by a complex, alluring and confusing person (Paul/the girl). It's a non-generic, specific, highly personal song you wouldn't have found on earlier albums. (You Won’t See Me is Paul’s reply to John.)
Whether you agree with these interpretations or not (by the way, instead of trusting my summary, it’s probably a better idea to listen to RS and the hosts in their own words), I’m happy to see the acknowledgment of the depth of John and Paul's relationship.
RS also makes a beautiful point about If I Fell (which, as we know, John saw as a continuation of In My Life): That John and Paul, as always, tell the truth about each other by the way they sing together.
(Cue the If I Fell/marriage vows quote from Gould’s Beatles bio).
Ian Leslie (no introduction needed) was more direct in his “Hidden Gems” episode on @onesweetdreampodcast. He stated he believes that If I Fell was written for Paul, commemorating their Paris ‘honeymoon’.
And look—people are free to go as far as they want in how they interpret all this, but I personally feel it liberates and elevates the discussion of their songwriting and relationships to include the romantic love or friendship or X or [redacted] or 'tender and tempestuous' but ‘not sexual as far as we know’ relationship between John and Paul as one of its many possible inspirations.
It just feels silly to me to ignore it or act all offended at the mere suggestion.
And when RS writes in Dreaming the Beatles “For John, Paul was the boy who came to stay; for Paul, John was the song he couldn’t make better,” it just feels right.
P.S. When I'm inclined to accept that Girl is about Paul, I immediately want to ask follow-up questions. Because this is a song about a fraught relationship, right? In what sense did John try to leave Paul? In what sense did Paul promise him the earth and cry? I know it doesn't have to be literally true, but some extrapolation, please? This didn't happen in the episode—obviously, since its focus was another song, In My Life.
PPS: I wrote this in a bit of a hurry so feel free to get back to me for clarifications, etc.