I feel pretty dense, but I cannot see what the big deal about this ad is. I realize the person in the center is pahllic in nature, but is that it? Can someone fill me in?
It’s an ad for lubricant.
There are somewhat-large posts spaced evenly along the pier.
She’s sitting where there would be another post.
I think she’s been filled in already. Now, are you?
Crikey! Now I got it.
Tee hee hee! This is brilliant, lno. 
…except that she’s sitting behind where the post is, and possibly cross-legged. To me it doesn’t look like she’s sitting right on top… though I’m sure what the joke is.