Soft porn of it's day

The book and film turned on numerous serial killers .

"The book and film turned on numerous serial killers ."

That's sad to hear. Looking that part up on Wikipedia, it does appear to be true. It reminds me of the Paul Newman movie Hud, where Newman said teens and young adults would go up to him, starstruck, and tell them that that Hud was their hero, because he looked cool and got women. Newman realized these young boys were idolizing the wrong character and completely missed the point of the film (the character Hud was a selfish monster and abuser...the Rebel without any Morals).

But as for The Collector, OP, I don't think this movie is nearly as lurid as those other exploitative films. This is still William Wyler after all. Yes, there's a scene that's close to showing topless nudity, which was very risque Wyler film at the time (but times were changing in the mid 60s to 70s), but even that scene was tastefully done as can be.

When Stamp's character was more amiably composed in the film, there's almost a literate quality to the film, and a certain sweetness to it (Maurice Jarre turns in a nice score here as well). Like Rope, Sleuth, and Dial M For Murder, I think The Collector is an interesting experiment in telling a story with only 2-3 actors the entire way through. I didn't get the feeling that this was an exploitative sleaze-fest. Most teenagers would've actually been bored with all the talking going on.

It's years since I saw the film and perhaps I remember the book better. But there is a very dark theme to this story and I find it quite believable that some would latch onto that aspect.

I guess it depends on your idea of porn or what a turn on is. Anything can be porn depending on your inclinations. As far as the film goes it is near boring in many ways, the book is more interesting but still far from what I would call porn or trashy.