Do you think Lupin and Lily Potter were romantic?  

I wonder if James thought that maybe Lupin was the traitor or maybe Sirius did. Sirius persuaded The potters to choose Peter after they had wanted to choose him ,Im guessing Peter put doubt into sirius's mind concerning Lupin.
I don't put much faith in some of the lines of the movie as they are the script writers words not JK Rowlings.

I have a question. Where does Lupin fit into any of it? He was never going to be secret-keeper for James and Lily.

Sirius Black was James's best friend and everything. They had wanted Sirius to be their secret-keeper. They switched at the last minute because he begged them to. He thought Voldemort would come looking for him as a likely suspect as the secret-keeper. He never knew that Peter Pettigrew worked for Voldemort. And, until Harry's third year, niether did Dumbledore or Lupin, etc.

Where did you get your theory from?