If they are set to exploring, and they flirt with or accept flirts from one gender, they will gradually develop a preference for that gender. If they are set to not exploring, they will only ever accept romantic interactions from their selected gender, or none if you prefer an aromantic sim. In the past the default for premades and townies was exploring, so that the player could start a relationship with any sim they wanted.
There's no faithfulness check at all, so the chances of your sim accepting random flirts from random sims is unchanged, except that if you set them to not exploring they will always reject any gender that is not checked as preferred for romance. If they are set to exploring and accept a random flirt, it could affect their preference.
Edited to add: As I understand what has been announced anyway. I am not sure what current bugs may affect the actual results.
