Companions could develop relationships with other companions, that is if the PC isn't making a (Successful) attempt to woo a character involved. (It's still possible to keep the Playersexual nature of companions so that players can still romance the character they want... But it's not a clear cut case of the companion will only accept the player and that all the player has to do is eventually pick the right options to enter the romance)
Having more failstates for relationships and companionship would also help. Often such things are exceptions (Like in WotR if you become a Lich all the good aligned characters leave you). Having more acts that cause a companion to end a relationship or simply leave the party (Possibly both - A common reaction to a break up is wanting space from the now ex-partner) would create more sense of the companion having their own thoughts and opinions over the players actions.
That was actually the norm for RPGs until the companions were turned into a major selling point (Mass Effect) which caused the devs to not have fail states anymore and each companion being a willing puppet without their own agency.
Just compare BG2 where NPCs would leave for various reasons, reject your advances when you said the wrong thing and even developed relationships with each other, making romances impossible if you were too slow with companions in new RPGs, including BG3 where they sit in camp forever and wait for you to interact with them.
