But... NPCs do sort of romance each other? We all know what happens at the party, and there's flirtatious party banter on top of that.
I'm pretty sure there is a difference between flirtatious banter and a romance story arc.
I will also point out that an NPC Tav/custom character is clearly not going to be romanceble, since what you have to do to romance a character depends upon their personality, and a custom character, by definition, has no pre-set personality beyond what a player gives them.