My theory is that there’s some past case that went badly wrong - maybe from Nightingale’s perspective it went only slightly wrong but from Seawoll’s perspective it was a total, utter desaster - and Seawoll’s just very, ahem, down to earth in his language, so I don’t think there’s more to it, it’s just a nasty remark. Nightingale as a rule doesn’t show any reaction to insults, so Seawoll just goes for a general insult, even though he tries to make it sting. I’d disregard the homophobia angle because according to one earlyish Q&A, BA reacted somewhat surprised that the tiny fandom reads Nightingale as gay, so if Nightingale was (at that point at least) not supposed to be gay it’s probably not supposed to be a homophobic insult. (Though I don’t know if there even are enough women higher up in the Met for a male DCI to sleep with to get advantages). Also, we know Seawoll is not led by prejudice in choosing his valkyries, despite often doing a convincing Gene Hunt impression. Plus, Seawoll values professionalism above all, and Nightingale doesn’t care much for Met professionalism - not even consciously, it’s just something that passed him by (which is why when Peter manages to get Nightingale to work more according to the Met’s current principles, Seawoll becomes a lot less hostile towards Nightingale). So I tend to interpret this remark as Seawoll being hostile as fuck, either on general principles or due to past bad experiences (or most likely both).