of course there are a million and one things that go wrong in the "man on fire" fight. it's miscommunication city over there. but truly the real downturn of the fight is when stede starts being mean about ed's fish and admits outright that he only ever complimented it in the first place to make ed feel good about himself. the thing is that it was obvious to the viewer that's what's happening; ed is not a fisherman and he caught one teeny tiny fish after an entire day of fishing. it would probably be obvious to ed too, and were he in a better place in general that wouldn't be a bad thing. because that's what you do in relationships! it was also incredibly charming for stede to be animatedly invested in ed's story regardless of the fact that the fish isn't impressive — he just fucked up severely by throwing that truth back in ed's face.

it's a crux of their relationship in the same way ed's investment in the treasure hunt in "this is happening" is. like, the treasure hunt is silly, the map is not real, the experience is just tramping around in nature all day for nothing, and ed truly dgaf. the main thing lucius tells him is that if he doesn't give into the silly and have a fun time with stede their relationship just straight up will not go anywhere because this is stede showing ed how much he likes him. so ed decides he's going to have a good time for stede. everyone's happy. their romantic relationship builds.

they're both relationship turning points, one that succeeds and one that fails, and they're both from the same source (which is the source of all their relationship conflict): stede thinks that ed needs adventure and that he, as ed's friend and partner, should be the rugged adventurer type to keep ed's interest. meanwhile, ed just wants to chill out forever.

i also think it's significant that as a father, man of whimsy, etc stede has always probably been more naturally good at responding to bids for connection — he's just the first to turn it around and throw it back when his feelings get hurt. but ed, whose character arc has in many ways been about opening himself up to whimsy, had to learn to respond positively in First Serious Gay Relationship 101 (taught by Lucius Spriggs). but it's very clearly a meaningful perk of being in a relationship to him, so to have such a new and good thing ripped away from him is literally thee fatal blow of the whole thing. he says "if you're not taking my fishing seriously, fine" and then walks away for good. my man was right ...... it is everything about fishing!!!!