Meh, Fiona was pretty okay with getting a handjob from Ashe before she even knew she was a duplo, so I really doubt its a sexual orientation issue. Ashe plays an important part in both variations of the sleepover scene actually, Ashe either gets Fiona off with just a handjob (which indicates Fiona got extra aroused by the fact it was Ashe giving her a handy, since otherwise it takes a skilled blowjob from Zoey) or even in the scenario where Zoey is the one sucking off Fiona what sets her off in the moment she starts cumming is making eye contact with Ashe.
Either way, I think it is more likely that Fiona's initial kink was actually the fact Ashe is her best friend, and Fiona makes a whole point in the latest update about how she tries to keep friendships and romantic relationships apart, so involving Ashe, who is her best and kind of only friend, in her sex life had a very big "forbidden fruit" factor to it.
Maybe she has a bit of a kink for the whole hermaphrodite physiology, but I actually think the main reason she's doing this whole "duplo admiration" thing is because Ashe is a duplo. She isn't attracted to Ashe because she is a duplo, she is attracted to duplos because Ashe is one. The revelation that Ashe is a duplo is how Fiona is internally justifying expressing her feelings of attraction for her best friend: "the revelation that she is a duplo changed things, it is okay for me to think Ashe is hot, because duplos are hot, so it is not weird for me to feel attracted to my best friend".
I think the duplo fetish is just an outlet for the repressed feelings of attraction she already had but kept buried to preserve their friendship.
Also I'm sorry, but if Fiona of all characters isn't an LI, then who is one in this game? Not much left outside of the incest plots and Valeria, and even that is still barely warming up...
Also I really doubt Fiona won't be okay with getting penetrated, she will probably be awkward about it and it probably won't be something she prepared herself very well for, but she really doesn't strike me as the kind of person who would make an issue out of that if it means pleasing her partner.