[...] allowing the player to just flip a "bi" switch if you want to romance them works against character development.
This is it exactly. This is what I want games to categorically avoid doing. We seem to have somehow lost the main point of a role-playing game and are veering off into some sort of dating sim here where everyone is bi so every companion can be romanced by everyone. And even then it is still unfair, because even though a bi character works fine mechanically for a gay/straight player, bi is romantically not at all the same as a gay/straight preference.
