Is there a way to complete romantic interaction daily task without romantic relationship?

You don't need a girlfriend or a spouse to do romantic interactions. Just find a prospective sim and do them.

Afterwards, ask to "just be friends" to clear the pink bar if it bothers you. Personally I'd just keep a few sims around town where there is some romantic relationship built so I have ready-and-willing targets to accept my daily tasks with less fuss. As long as you don't ask them to be your girlfriend, they are just sims that dig your sim...no more no less.

@Stormkeep Thanks for the reply. What I'm worried from doing romantic interaction with other sims is they are still remained in the heart tab of the relationships even after they are remained as friends. I don't know why but I'm annoyed seeing the npc's name on heart tab in my sim's relationship window. I'm not sure relationship window can be a proper name and can be understandable for everyone here but I don't know how to call this. Relationship panel? or window?

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For my other actor sim, practice pickup line at the mirror worked for researching romance emotion task in acting career. I can try the same thing for my secret agent sim if it works.