Well, Charade also says she was told that Aveline was sleeping with Hawke. I think people just love to make up stuff about her. Or maybe one of the guardsmen indeed had a crush on Cullen. Which I can totally understand
Cullenites in Kirkwall! 
That's what throws me off. Aveline is a guard captain, although I suppose Varric wouldn't have been so obvious as to use the title "guard captain" because she would have caught on, lol. If anything I found her to be pretty rude to Cullen when he was just trying to prevent her from losing her job to him thanks to Meredith.
It's the part where he says "I have an idea... but the story..." In reply to her asking how it ends, it was suggestive that the former was a real thing and the latter was based on it, which he then confirms by saying that it was always there just waiting to be uncovered.
So either Aveline had an unrequited crush, or Cullen had a girlfriend/boyfriend in the Kirkwall guard. I'd prefer it to be the former, I don't want another Thane/Carth Onsi/Sky situation on my hands!