Can you see any way in-character Jegulus could happen? Or hypothetically if you *had* to make them get together (pre- or post-Voldemort pinterest board), how would you do it and what would their relationship be like? And how do you think Sirius would react?
So for me, the most random ships can work if the characterisation still feels similar to canon. And while we don’t see Regulus on screen, there’s enough to extrapolate from and build a picture of a passionate, slightly obsessive, very proud, but insecure person.
And we also have enough to build a picture of James as a coddled, brilliant, privileged boy who had a soft spot for under dogs but could also be cruel and arrogant.
I think Jegulus could start as enemies to lovers where they fight about the war (but are really fighting over Sirius’s love) .
And they end up having muddy, hate sex on the quidditch pitch after having a dick measuring contest and a punch up during a quidditch practice.
But the key stumbling block to a full relationship isn’t Sirius clutching his pearls, it’s their diametrically opposed world views.
According to Kreacher, Regulus loved to yap about how much he wanted to dominate muggles and muggleborns and bring wizards out of hiding. And while James is atrocious during SWM he literally says he would never call anyone a ‘you know what.’ So he can’t even say the word, that it is safe to say (based on extrapolations from canon) Regulus was throwing around casually.
From what we can glean, both of them are passionate and hold these views so strongly that good dick game is not going to persuade them to fundamentally change their morals.
As for Sirius’s reaction I think he’d be perplexed that they fucked and maybe he’d be upset because he wants a separation of church and state (his family from his friends). But I don’t see him being ‘dramatic’ about it or caring about his delicate brother’s honour.
And that brings me to my final point, from what we know of Regulus he is far more like Bellatrix than anyone acknowledges. He wouldn’t be the passive, delicate one in the relationship and when this is paired with James being desi this gets into weird colonial fantasy territory.
Put it like this: if you are a MAGA Trump supporter or a right wing fascist in Europe, it wouldn’t matter how gorgeous you are, that would just be a hard no for me and James Potter when it comes to a relationship