Crossguild HC
When Shanks finds out about Crossguild, he's actually convinced that his Exes got with Crocodile in a poly relationship and is devastated.
The entirety of Crossguild is doing their absolute best to make it clear that the day they fuck any of the other members is the day they lose every ounce of their meager self-respect, but Shanks keeps walking in on them in 'odd' scenarios and any progress they had with convincing him is ruined instantly.
At one point he walks in when Buggy has lost his temper and is yelling at Crocodile a bit, his limbs floating. Shanks is instantly brought to tears and is like 'Oh my god, you're yelling at him while shaking your disembodied limbs angrily...you used to do that to me when we were together, I guess you've really moved on...' Crocodile and Buggy both just...stop and share a look of mutual look of 'What the fuck is he going on about?' before being disgusted that they had the same thought, which only proves to further cement them as a couple in Shanks mind.
I need to make it as clear as possible that for Crossguild, the thought of dating each other hadn't even crossed their minds. They'd honestly be fine with faking it to fuck with Shanks if they weren't so uniquely horrified at the idea of dating each other. This can end in a slow burn where they actually do end up getting together or them just getting slightly less homicidal (and gaining some grudging respect) to each other cause
'You know clown? If I had to grow up with that red motherfucker like you did, I'd kill everyone on that ship. Respect to you for not doing that.'
'...thanks? I guess Crocy. And you know, Mihawk? If I was married to that bastard, I would've actually killed myself so honestly respect for not doing a murder-suicide.'
'Thank you, believe me, it crossed my mind.'