I believe the need to interfere stems from an unsatisfactory self. People are constantly upset or frustrated with where they are at in life, or second-guessing themselves, or comparing themselves to where others are.
Like you say, the matter of indecency. Obviously I reserve my judgments when I see it in public, but I don't feel the need to project them. I personally feel nobody should be subjected to seeing it, but if it doesn't affect me, I don't comment.
That is my philosophy for the most part, "if it doesn't affect me, don't pay attention." I only wish others could do the same
This is one way to look at it and I agree with you. Some people project because they themselves are dissatisfied.