This better not by shying away from the thruple ending. Although given the differences in the kisses each got, Emma is surely the favourite for a monogamous ending.
from what we've seen so far, it seems more like she's only (truly) romantically interested in men, whilst still being sexually interested in women (as well as men).
If this were to go for a monogamous ending, I don't think Emma would be the choice, since I think Mashiro doesn't and can't view her romantically, only sexually, which is why she got jealous and didn't like it when Gen was about to kiss someone that wasn't her