Mainstream in the sense that it is not a porn film. It was screened pretty extensively on the art house circuit, back in the day. You will usually find it in the ‘foreign’ or ‘Japanese’ section of video stores (as opposed to the porn section), though to be sure you have to look in stores that have an extensive foreign section.
It is also somewhere on the Sight and Sound Top list.
I’d agree that it is not mainstream in the sense that it will be screened down at the local multiplex or that it can be found on the shelves of Blockbusters.
Don't Look Now with Julie Christie and Donald Sutherland. It has been rumored for many years that they were actually having sex. But Donald Sutherland refutes it (he talked about it when he was on Inside the Actors Studio).