I think that would be a good addition to KOTOR3 (or any further game with romance in it). Make it somehow affect the story.
For instance, as a male PC if there were 2 female NPCs vying for your affection, if you choose one and reject the other, then the rejected one maybe leaves the party, or even worse, betrays the party at an opportune time. Or to a lesser degree, if a NPC has some information that could be useful for solving a specific quest, if you fail to romance her properly, she just never reveals that information, making you have to complete a quest the "hard way"
Example: female NPC knows the codes to a secret passage way to a control room. If you romance her, she'll give you the codes, you complete the quest to find a way in the base. If you fail to romance her, she won't give you the codes and you'll have to then fight off the guards and find clues on how to make it to the control room the long way.
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that would be awesome.