and "Wham, Bam, thank you Sam" type stories tend to be much shorter. They tend to lack plot. They call those PWPs (Plot? What Plot? or Porn Without Plot)
I have written a few stories that I intended to be porn-like, but I still have more charactization then you'd generally get in the "hey I think you're hot, let's f**K" stories.
I have also tried breaking the mold a little in experiments in writing. My Colin Creevey and the Parliament Dreams spin-off... I pretty much start off with some graphic sex... then there is a long chapter afterwards without sex.
great question Comsie