Actors have the often difficult task of making everything they do on film appear believable to an increasingly savvy and cynical audience. We're going to be able to tell if they're "acting" too much, which means everyone on set has to try to capture authentic performances any way they can. ...Which can make things a bit awkward when it comes time to film an intimate love-making scene.
How do you make people believe that these two (or more) characters are really going at it? Especially when the circumstances on set are less than ideal? Because contrary to what viewers might think, those scenes aren't usually performed on a closed set with just the actors and a couple of camera operators who have their backs to the action.
No, usually there are at least a dozen crew members standing around, holding boom mics, adjusting the lighting, and applying makeup between takes. It's a full house. And even the most professional of actors is going to have a hard time performing in a situation like that.
Which usually makes for some interesting behind-the-scenes stories.