We want to show the ugly dude getting laid by this hot porn star so the rejects of society think they stand a chance.
I know, right? How about a little reverse-action on that premise for the ladies. The hot fantasy dude with a normal looking girl, i.e. no Barbie parts.
And, yes, I know the target audience is men, but I think it's because they just don't bother to target women. If they were smart, they would. After all, George Clooney and Brad Pitt didn't make it to super-stardom based solely on their acting abilities. They made it because large numbers of women paid to see their flicks.