You can add me to the list of people who used to think this way - it seems it's a pretty common aro experience to think that "romance" just means "good friends who have sex"! I also started to see the flaws in that definition when I learned about asexuality and the fact that many aces have romantic relationships.
My understanding now is that what people call "romantic attraction" is (probably) part physiological phenomena, part subjective conscious experience of physiology, and part social framing of subjective conscious experience.