I've been thinking about this exchange between Izzy and Ed in "Discomfort in a Married State":
"For years I've followed your every whim, I've managed your increasingly erratic moods, I've massaged this crew when they were worried about your judgmentā"
"Mm, sounds stressful, Izzy."
Izzy has assigned himself the role of "managing" Blackbeard, but it's quite obvious, both in Ed's response and in the way he says it, that Ed has never wanted or expected him to. And he continues to try to manage Ed, even when Ed clearly tells him not to.
Both Ed and Stede have that word "whim" thrown at them by people who don't really understand them - Izzy and Mary - and both at first resist the word (Stede even says "I object to the word whim"), then internalize it. It seems what are being called whims by outsiders are actually expressions of deep desires that neither Ed nor Stede have the verbal or emotional language to describe.
It's not a whim that has Stede running off to a be a pirate - it's all the frustration and loneliness and repression - and it's not a whim to go back to Mary - it's a result of guilt and self-loathing. It's not a whim that has Ed following the Revenge to meet Stede - it's wanting to find someone who might be a kindred spirit. But neither of them can openly articulate those feelings, either to the people who are calling them whims or fully to themselves. Stede goes further than Ed does because so many of his desires are located in his repressed homosexuality, and once he's able to articulate that - with Mary's help - he understands his feelings. He still believes that he's "whim-prone" in other ways, but not when it comes to Ed.
Ed is especially leery about his own desires, which have been managed by other people for so long, and about Stede's. His idea to "run off to China" is a whim in a certain sense, but it's expressing a desire to leave behind their old identities and form something new together. His desire to "take it slow" is about his own healing independent of Stede, but it's also built on a fear that Stede is going to disappear again. After they have sex, Ed again falls into the fear that it was a whim, but it's not his whim - it's Stede's. He's scared that what meant so much to him doesn't mean as much to Stede, or not enough for Stede to be able to let go of piracy and fame. He's worried that piracy wasn't a whim, but he was.
Both of them have been managed most of their lives, in different ways and by people and structures that they never wanted to manage them in the first place. It was all about keeping them in settled, socially acceptable places where they can't escape, and casting doubt on the legitimacy of their desires. (I don't think Izzy, and certainly not Mary, consciously think of it in these terms, and Mary especially has had to subsume her own needs as much as Stede has.)
In the scene with Izzy, Ed's evidently pushing back at the management, and he probably has been for a while. But this is likely the first time where his desires are getting more articulated, after his conversation with Stede, and are the same moment when Izzy starts trying to exert even more control over him.
His finding the letter is a mirror to Stede's conversation with Mary - the realization of who he is and who the man he loves is, and that the feelings he's experienced not just for but from Stede are not whims but bone-deep love.