I think you hit on a possible fix when you explained what you meant, replacing "until", which implies a situation dictated by circumstances rather than choice, with "unless" would help.

"Unless Naruto somehow turns himself into a real, functional woman and lets me knock him up, you're it. Shut up."

It could still be taken as Sasuke preferring Naruto but it has a lot more wiggle room since it doesn't imply that if Naruto did figure out how to get pregnant that Sasuke would no longer have children with Sakura.

"Unless Naruto somehow turns himself into a real, functional woman and lets me knock him up, you're going to have them all. Shut up."

That might work? It explicitly states that even if Naruto did get knocked up he'd still want Sakura to have his kids too.