I will be frank. I don't want the story to make Eri "Fuyuki sexual" if you know what i mean. Yuri as a genre LOVES that shit but just be realistic and mature about it for once. Than you could say "why make the story a yuri at all if they won't get together?", well, Fuyuki is, undoubtedly, a lesbian and this has been her story.
Eri does indeed care about her a lot but romantic (and sexual) love is something entirely different. Eri who fucked her boyfriend without a second thought (and given absolutely no thought about Fuyuki "in that way", not even romantic way) suddenly becoming bisexual for the sake of them getting involved romantically moving forward doesn't really sit well with me.
That's fair.
My take on Eri's deal is that she's very reactive/passive when it comes to relationships. I don't think she had ever confessed to someone; it had always been done to her. So if she doesn't really get romance, and relationships to her are all experienced reactively, then stuff like sex would just be "what is expected of her", and not something she's actively seeking.
When she did sleep with Kyo, it was off the back of being mad at Fuyuki for ditching her on her birthday for Fuyuki's senpai.
Eri also had her conversation with her seatmate during class where she'd asked if it was proper to prioritize lovers over friends. She was likely asking, trying to put Fuyuki's change in behavior into perspective (was Eri being de-prioritized for the first time in forever, from her perspective), and she resolved that if that was the case, she'd have to accept it.
So when they had sex, what Eri
didn't know is that Kyo asked Fuyuki to lie and give him space with Eri that day. I don't know if he then initiated the sex and Eri responded, but to me her having sex with Kyo was trying to prioritize her lover relationship, and she had no way of knowing at the time the truth behind the whole setup beacuse it was hidden from her.
So to me that doesn't say "Eri is suddenly bi-/Fuyukisexual because Plot", that just says "Eri doesn't understand her own sexuality and always relies on the cues she thinks she gets from others". She says herself in this chapter that she can't read the room, or read the soft tells of others, and has to rely on tangible things to know what's happening--which took the form of passively waiting until someone else moved, and then reacting or reciprocating as she thought was best in that moment.
But with Fuyuki, she's active in her pursuing her. Even as Eri does everything she can to get Fuyuki to come running to give her time and attention, she's still the one performing the action, to reach out to Fuyuki, and has been all this time.
So while she might never before now have though of Fuyuki in a romantic or sexual manner, Fuyuki remains the one person in her life she's been proactive about when it comes to their relationship. That is such a marked difference from everyone else, that I suspect a relationship that progresses through all the intimacy levels would make sense for them, even if it didn't happen at the outset.
Hence why I think that even gradual romantic or sexual feelings between them could conceivably develop, and possibly in a way where Eri isn't just "reacting to her partner of the moment"--especially as she's resolved to make the changes in herself necessary to never hurt Fuyuki again in the way she has.