Difficult question, even if you ignore the problem in defining what is meant by "pornography". I would think that you'd have to eliminate the type of explicit material that involves children and the exploitation of someone against their will, both of which are obviously wrong. Some people would eliminate depictions of homosexuality and bestiality on a similar basis. You can go on to limit the field in all sorts of ways.

If you limit the field (on whatever basis you choose), and then change the question to "what's wrong with pictures that arouse the libido?" I can't give you a good answer. There are many prohibitions against practices that effect only the individual that are designed to protect us from ourselves. Maybe there are reasons rooted in religious and social taboos that may or may not have been legitimate once and may or may not be legitimate now. Maybe there a real, sound psychological reasons (the possible substitution of imagination for reality, or possible withdrawal from actual human contact, etc.) for these taboos. Personally, I think it's all extremely subjective.

If it doesn't become obsessive or excessive - why not?

Bart