I am a book reader much more than I am a movie viewer. I'm always scared about the result, since usually movies don't come close to reproducing the imaginary of the author nor does it represent the imaginary of the reader by a long shot. I was always disappointed when watching a movie that took its source in a book I've read. So I rather watch a movie about a book I have yet to read, and if I like the movie, then I know the book will be much better, so I'll read it. I always go from movie to book, never from book to movie, even if the cineast has a reputation of making wonderful movie pieces.
This pretty much sums up my opinions of books vs movies. I'm not saying I have never really enjoyed a movie of a book, but as nilstreet says, if it's a well written book you already have the imagery in your mind and you don't need to see it acted out on the screen - sometimes that can spoil it.
Films of books are, of course, a means of making money from someone else's idea. They also offer a comparatively fast way for the viewwer to assimilate the story. You have to put more effort into reading a book, but I find I get more out of it too.
A rare example of a film BASED on a book (a short story actually) where I thought the movie was better than the book, was "Total Recall". It was based on "We Can Remember It For You Wholesale" by Philip K Dick (stop sniggering, that was his name, I did not make this up). The idea behind the story, planting artifical memories in people's brains, was taken as the basis for the film, but the actual plot was changed quite a lot, exploiting the possibilities of the film medium, and in doing so I think it actually improved the story. I have no problem with that kind of movie.
Science fiction usually translates particularly poorly from book to film, because so much of it involves weird concepts that are easier to describe than to make into visuals, and some atrocious science fiction movies have been made over the years which I think have given SF a bad name in some people's minds.
As for gay books (sorry to go right off topic here), I wouldn't mind seeing one of John Rechy's novels made into a movie. I would prefer it be an R-rated movie though - if it was X-rated, the distribution would be restricted to the porn market, and I am afraid it would be a case of pearls before swine. No offence intended to anyone here who watches porn, but while his books are full of sex, there is so much more to his novels than the sex.