DISCLAIMER: THIS IS GRAPHIC.
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.