First of all, in the US at least, the X-rating isn't around anymore. It's NC-17. And if we're talking about real movies and not porn, then an NC-17 rating is pretty much a kiss of death, since it means most stores and theaters aren't going to show it or stock it.
In the US, most
erotic films (in contrast to porn) tend to be released "not rated" rather than submit and receive an NC-17 rating. This goes for a lot of non-erotic films that just happen to feature sexual situations that are too graphic for R (
Requiem for a Dream is a good example of that).
I'm not entirely sure what you mean when you say "X-rated film" since it's been forever since any film's been released in the US (I'm not sure how foreign ratings systems work) that was rated "X."
However, there are some great explicit erotic films out there that aren't porn and do engage the mind.
I think it's quite a shame that the MPAA rating system is so fucked up and place so much emphasis on sexual content over violence, or that so many places refuse to stock NC-17 movies. There aren't nearly as many good erotic movies out there as there could be.
In any case, a great erotic film is
The Dreamers. (Don't bother with the R-rated version; get the NC-17 one.) Did I mention Eva Green?
ETA: And of course, right before I post Wayne has to go and find an actually-X-rated film....