For good reason, at least.
For all the talk about duration, position, and size, one of the often overlooked facets of fornication is verbal communication. More specifically DURING sex. Because nothing kills the mood quicker than saying something so bizzare that you wonder if people should actually take a U.S. citizenship-type test before engaging in relations. It's one thing to know what you're doing anatomically, but the verbal game needs to be on lock as well. In other words, be tactful for crying out loud.