Porn or erotica: does it matter if you're 4?

A strange question, and not one I anticipated having to grapple with on behalf of my 4-year old son quite yet. I have been mulling over this one over for a few weeks, wondering whether this is actually a problem or not.

I went to collect my beloved innocent boy from a friend where he'd been to play for the afternoon. His mum welcomed me into the house and took me through to her bedroom where the boys were watching a dvd. While listening to her natter away about her renovation, I chanced to look up at the pictures on her wall...and choked.

On the walls were six large framed photographs of very naked women. They were not Rubens-like old fashioned black and whites. Nor were they culturally interesting and beautifully rendered drawings of couples shagging a la Karma Sutra. Nope, these could not be described as art. They were full colour full frontal fully nude shots of women with NOTHING left to the imagination. They were porn. Granted they were not ugly porn, with girls called Kimberley displaying shaven air-brushed pudenda and captions underneath saying things like: Kim likes to ride horses and read poetry. She also likes her men hard and ready!

These did have a certain appealing aesthetic. Some might argue that the pictures constitute erotica. The liberal in me demanded they be classified as erotica. Indeed I may have argued that until I had to think about my baby boy looking at them whilst watching Monsters Inc with his little friend one quiet Friday afternoon.

What do mothers do in such cases? They phone another mother. Early the next morning I rang the mum who is my arbiter of all things parental.
Me: "Has L been to play at T's house?"
Her: "Yes."
Me: "Did you go inside?"
Her: "Yes."
Me: "Did you go into the main bedroom?"
Her: "Oh, you're calling about the pictures?"
Me: "Yes."
Her: "Of course. T's mother showed them to me specifically. Said that's her and her husband's thing."
Me: "Oh. Nice for them. Do you think it's OK for our 4-year olds to see them?"
Her: "Yes."
Me: Oh, OK."
Her: "Did he mention it to you?
Me: "No."

So I dropped it. Then I took the coward's route. I invited the son of the sexy mummy to play at our house rather.