Since I watched film educational documentaries at school, I can't even imagine what material was used before
Have you seen movies with actual birds and bees (before they switched to prosaic same-species material) ?
Not quite only sex docos, but there was a doc called 'After Porn Ends' which was quite good and a series on Channel 4 in the UK years back called "Dark Side of Porn' which was fascinating as well.
A very interesting one to watch is Louis Theroux's Twilight of the Porn Stars, which delves into the effect Internet sites have had on the porn industry.