If there wasn't anything going on with Jamie and Dakota while filming Darker, do you think they still would've had Jamie suck on Dakota's boob in the shower/red room scene? I feel like that was a bit more intimate than the other sex scenes and it'd be weird to do if Dakota and Jamie weren't together.
Such act certainly is something kind of intimate as you say but is not the first time is shown on films even series and many actors have done them. Given the content and genre of the Trilogy a scene like this is not something unexpected or surprising or weird to be asked by a director to be honest.
I’m sure both J&D are professionals and if asked to do it(even if they weren’t together) they would be game on. Plus both have said that they are ok with such things(considering they do see a valid reason a scene like this to exist) and since day 1 they knew for what they signed for.
But the most important factor in my opinion that makes me certain they’d go for it nevertheless is the trust they obviously have with one another. Friends or couple …trust is trust and respect is respect. They feel comfortable with each other they share humor which also helps them go through moments that might feel a bit awkward.