I'm curious: Have you ever seen In the Realm of the Senses? A lot of critics revere that, as a study of sexual obsession. It's a lot more sexually explicit than anything you've mentioned here though. :)

A couple of comments on a couple of films you mention:

Fifty Shades of Grey: There are far better films about BDSM. Belle de Jour, Secretary, and The Duke of Burgundy, for instance.

Showgirls: I know it has a cult following, but I've never understood the appeal. I'm a big fan of Paul Verhoeven in general (including early provocateur works like Flesh + Blood, his famous sci-fi films Robocop, Total Recall, and Starship Troopers, as well as later underrated gems like Black Book and the contentious, but darkly hilarious Elle) but Showgirls has always struck me as rubbish. Regarding the nudity, I recall legendary film critic Barry Norman commenting in his review that there was so much of it during strip scenes that he had to fight the urge to yell "Get 'em on!" (ie instead of "Get 'em off!").