Thanks again, R52, for your comments. Lots to digest!
I figure the other commenter was suggesting that Starting Over is about Paul rather than Yoko. I tend to agree, though I haven't always thought that way, simply because the style seems designed to appeal to Paul (in the same way that parts of McCartney II seems designed to get 'at' John). There's the introductory part ('our life together...'), which was a songwriting trick that Paul has said they both liked, from the sort of pop standards they bonded over in addition to rock and roll, when they were just getting to know each other. (Any song that includes this makes me wonder (tinhat time, forgive me) if they wrote it for the other -- If I Fell, Here There and Everywhere, Wild Life, I Know (I Know), etc). And then the song as a whole, the Roy/Elvis/Gene Vincent thing John is deliberately channeling, is the sound of 1959 or 1961 -- a time that would've been nostalgic for Paul and John, not for John and Yoko. Besides, Yoko had total disdain for that sort of music (‘wallpaper music’, in her words) so it's hardly a romantic gesture for him to write it for her.
The 'in bed' comment (which, yes, Paul did actually say!) is the sort of thing that makes me doubt that anything happened, just because if they have something to hide, they sure seem to let it all hang out. There's a straight-forward, acknowledged romanticism in the way they refer to their relationship (as a marriage, a love affair, etc) that seems to belie the idea that it went beyond the psychosexual pseudo-romantic we all know about, and into actual sexual affair territory. Why wouldn't someone, somewhere have leaked or broken this story, especially with comments from them (like 'in bed'), which practically dare someone to do so? For a long time I just doubted it was possible for them to command whatever combination of respect/fear/sympathy/loyalty to keep everyone keeping this secret for 40+ years, so I was thinking nothing more than what is already publicly known could have happened. But all the Weinstein, Louis CK, Kevin Spacey, Jimmy Savile, etc stuff has me rethinking. All of those guys had rumors for years, sometimes referenced in public (even by them!), yet those scandals weren’t openly spoken about until whatever recent combination of circumstances broke them open. So I guess I'm leaning more towards the 'something happened' side than I used to, simply because it seems more plausible that they could've kept it secret.
(Also, I figure that it’s likely that some people at Apple and/or MPL are aware that a substantial part of the most highly active young female fanbase is into the idea of John and Paul together...and Paul is maybe even aware...and by dropping hints while his official line is ‘I never saw even a hint of gay from John, no, sir’, he gets to appeal to all)
As for when/if Paul would've first spoken to Linda about it, he's said they had a brutally honest conversation about sowing their wild oats sometime before the married. Since Paul seemed to believe that John and Yoko had done something similar, and that he would've come up on their conversation (“I understood what happened when he met Yoko. He had to clear the decks of his old emotions. He went through all his old affairs, confessed them all. Me and Linda did that when we first met. You prove how much you love someone by confessing all that old stuff. John’s method was to slag me off."), it seems very likely that John would've entered into Paul and Linda's discussion. As for Linda being accepting, there’s been vague innuendo for years about Linda and Lillian Roxon, including in the recent Jann Wenner biography. So Linda may have been okay with it for many reasons.
Anyways, a few more questions for you, R52, if you have the patience to answer: what have you heard in particular that makes you think their relationship was sexual, rather than just non-sexually codependent and deeply enmeshed? How do you think the nature of their relationship has been kept secret for so long, and do you think the story will ever ‘break’?