I rarely see sex portrayed in the media as fun. Mostly it's Very Serious. It's Angsty or Romantic or Violent or Hot. Which is cool. Those are all A+.
But I don't often see two people laughing during sex*. I see smiles and grins, but they're usually Doting or Flirtatious or, occasionally, Evil. There's always a very These Expressions Are Exclusive to Sex feeling to them, if that makes sense.
The last time I saw something like fun during sex, amusingly enough, was in the third Takumi movie. I'm not sure if it was intentional or not, but one of the actors has this, just, happy smile on his face, like it's fun, and it makes the scene that much sweeter and more intimate.
Because sex can be awkward and ridiculous at times, so I love the idea of two people being so comfortable with each other that they can laugh about it.
Random thought as I write a sex scene. XD
*When I do, it's usually on this tumblr. :D