Sex standing in back of pickup truck

[Robot_Shark]

I read this story years ago. I can only remember that at one point fairly early in the story, a co-ed group of college(?) friends head out on an overnight trip to a beach or campground. They all ride out in the bed of a pickup truck. The female narrator and her friend decide to make the trip in just tiny bikinis. Her GF, whom she describes as being more of a slut than she is, lets one of the guys get her naked before they're even out of the city, and she ends up standing, holding onto the roll bar, getting fucked from behind while they're surrounded by traffic.

The overall theme of the story might not have been exhibitionism (s I recall there was some other stuff that really didn't suit my tastes), but I thought that scene was damn hot. Searched several times and I can't come up with it.