DISCLAIMER: THIS IS GRAPHIC.
Remember in health class when they taught about how sex works, but they really didn't explain anything so that you actually know what's going on. Well, this video from a show in the UK (of course--the US couldn't get away with showing anything this real) called 'A Girl's Guide to 21st Century Sex' shows exactly what's happening. It is extremely graphic--it actually shows a couple having sex. If you feel uncomfortable watching that, do not watch this video. The only place I can find the actual sex/explaining what's happening scene is on a porn site. Other websites (like youtube) obviously won't publish the actual sex part. But it is interesting to see what's going on inside.