If I had to simplify Berger's thesis, I would say that there cannot be a complete objective interpretation of the image, "what it means" is context-dependent, where what's displayed, who's looking at it, what they experienced in life, and what mood they're in can change what they "see" in the image. Traditionally, image locations were relatively stable, so at least that aspect of "seeing" was controlled, but with the advance of technology, we can replicate images and embed them in different contexts with different purposes, and so art criticism and analysis should focus not only on the image
in isolation but also on
who and
how distributes it, because that part of art (where it's displayed, by whom, to whom) carries meaning in itself. As I said, he was an art critic, so he focused on the methodology of how art can be analyzed.
I really don't know where you got the idea that he was saying that one can divine some knowledge from a painting or that paintings and culture influence our sexuality.
Edit: Spelling and punctuation. God, I really should enable a spellchecker. I'm a terrible speller, I apologize.