The secrecy and forbiddenness doesn't have to be overt, but it's practically always there. I know I can't imagine any of my slash pairings being at all interesting in a world where anyone can have anyone they want, and all the society at large would have to say is 'mazel tov.' No obstacles, no need to hide your feelings, no need to feel like there's something wrong with you, and no intensification of feelings through repression means no massive existential relief/joy when your feelings are returned. It's just a nice little love story, boy meets girl 2.0