There's so much threesome talk. As a general proposition, if the MC and Haley are actually going to go ahead and get groiny, as I expect they will a few chapters hence, there's a very obvious and real-world-logical reason for bringing a third person into the mix: Haley and the MC would not be able to openly live together without suffering a lot of judgment and criticism. Taking on a third person is the way to beard the relationship, whether or not either Haley or the MC are having sex with him or her. Since the game is extremely thin on non-creepy men, it's natural to assume that the third would be a woman and the fake (or real if Haley swings that way and is open to the idea) public relationship would be between the MC and the other woman.
I really don't think that Sandra would be a good candidate for that arrangement, though maybe her personal assessment of her ideal work/life balance would enable it because she desn't feel like she could commit to a full-time relationship, but we've been offered plenty of alternatives. I really have no idea if the game will go in that direction, but if it did that's one reason why it could.