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.