I think your point about how people practice similar things with different motivations is a good insight.

Although most fantasy novels are based on premises which would be unpleasant if we were to actual live them out (A world of nobility and classes and enforced servitude? No thanks) they allow us to insert ourselves into lives we can only vicariously dream of, sort of like daydreaming of being a rock star.

I attempted to read Fifty Shades but got tired after a chapter or two and skimmed the rest. The difference between it and the classic works of the genre like The Story of O or the Beauty trilogy by Anne Rice is that Fifty Shades really was about the fantasy of money, and the romance was pretty much secondary.

Ana falls for the idea of having an endless buffet of goods and toys and power. But she really brings nothing to the table except her willingness and availability. It is sort of the reverse of those movies where a schlubby guy somehow manages to bag an impossibly hot model- there is never any explanation of why Christian (or the reader) would be interested in her.

By contrast, O and Beauty are more vividly drawn characters who are more easily believable as the object of fixation and devotion. More importantly, they approach the relationship more on the plane of equals. Their motivation is different, more on the lines of classic romantic tales than the trivial aspect of Fifty Shades.