You can literally choose not to start anything sexual with Fiona in chapter 4 so i find this whole complain about lack of player agency very disingenuous.
Also the developer never promised anyone that the routes would equate with a complete change in both Ashe and her partners sexual preferences. Morrigan even made a giant text explaining what to expect from the routes.
Just to be sure i came back to chapter 4 and checked:
if Ashe doesn't have sex with Fiona in chapter 4 then nothing happens between them in chapter 5. That is player agency happening.
So i am not the one using a straw man falacy to complain about something that was the obvious outcome of player's choice simply because said outcome happened a update later.
I think you mean sex partner there instead of object? Since there are feelings of friendship between both of them involved.
And while i agree Ashe having a pussy may likely have been what started that sexual attraction to what is unknown/different in a duplo, i would not discount the idea that in the future Fiona may bottom, sometimes, for Ashe. At least for a Siren Ashe.