When I lost my virginity, the movie on in the background was Tron.
Back in the day, Kevin and I almost had to watch a movie to have sex. It was relaxation, it was distraction, and it was almost the only way to make things feel natural. We couldn't just crawl in bed together, that would be like we were having sex on purpose, that's way too intimidating. Instead we had to play the "we'll just lie on the bed to watch a movie and see what happens" game. Usually we didn't get to the opening credits.
(During an awkward reunion several years later, we were watching the Evil Dead trilogy, I started getting all gropey and he uncomfortably told me he didn't want to have sex during a horror movie. With glorious obliviousness, I patiently waited out the first two films, then started arguing that Army of Darkness is really more of an action comedy. See, this is what you get when you make weird excuses instead of just saying you don't want something.)
I still like moviefucking as a way to keep things slow. When you're in bed just to have sex, you tend to cut to the action, and sometimes that's awesome, but it can be hard to pace yourself if you're in a mellower mood. Just lying there groping without getting to the wet bits can be frustrating, even boring--it'll feel like you're drawing things out.. Whereas telling yourselves that you're supposed to just be watching a movie, but if you get a little naughty, no one will know... even as a grownass adult, that's just fun.