The primary force at work in this is sex and, especially, voyeurism.
Hm. That's not my reaction. In my opinion Weston's nudes were more voyeuristic. However, I don't really like this example of Mortensen's. I don't like the missing head, the foreshortening of the legs, or the awkwardly twisted wrist.
Here's a list (from the Model) of types of Nude Mortensen thought best avoided: Nujol nude (oil-covered), mal de mer nude, prudy-nudy, dislocated nude, pretzel nude, speckled nude, blubber nude, pimpled nude, hirsute nude, abraded nude, sun-burnt nude, gooseflesh nude, hippity-hop nude, seaside-sunset-silhouette nude
"A sedate variant of this type [the hippity-hop nude] is the seaside-sunset-silhouette nude. The elements are fairly constant: one setting sun, one breaking wave, one yard of crepe de chine, one piece of seaweed, and one lightly tripping nude who is outlined against the sky and the shining sand. Sometimes she appears in a more meditative and mal de mer attitude, and sometimes the scene is enlivened by the presence of three lightly tripping nudes and three yards of crepe de chine." Model (1937) page 221.
, crepe-de chine nude, portiere nude, Tiffany nude, Knick-Knack nude, the Emily Post nude, Ah Me Nude (also known as the 'Welt-schmertz nude')