I'm not sure if canon Bran can have sex or not, just because it depends on how much control he has over certain muscles. It is possible for people who are paralyzed to have sex, but it depends on how severe/how the injury manifests and I don't think GRRM has given us enough info to figure it out either way.

Maybe there's less Bran fic because there are already male characters that authors would rather write about--like Robb, Jon, Theon, Jaime, etc.