I guess things can be argued either way but i'm not just coming from the angle of 'the other girls will take more time than works for a christmas present' but also from a game development perspective, Selly could've made the threesome a main event with variants for all 5 girls, covering his bases and still allowing for a 100% completion, but now that missing 2 events is unavoidable if you don't pick Ayane that seems impossible, there's going to be 5+ missed events no matter who you pick and that doesn't at all match Selly's usual method so either he's doing something brand new with this or the simpler answer is Ayane is canon and the other 4 are red herrings. I really am inclined to go with Ayane being the only one that results in an actual threesome cause it'd be typical of Selly to fuck folks over like this, he's demonstrated with previous flags that he considers there to be a true way to play and you'll be punished with missed events and other things if you step outside of it, which is exactly what has happened but even if we side step all of that, this isn't a sexy porn game, the sex is but a vehicle for his story, and i just can't see him creating 5 separate branches just for the sex, nor can i see him creating 5 separate branches for the story, that's so much more fucking work.
No, if you step back and really think about this from an objective view, consider Selly's style and the development history of the game, 1 possibility fits faaaar better than the other. I'm happy to be wrong, but i will be genuinely surprised.
At the very least i would recommend anyone who is unsure to keep a save backed up from before this update starts and continue on with one where Ayane was the chosen threesome partner, less risk of having to backtrack that way due to missed events. Could be over a year from now that we get confirmation either way.