Can't say this is dangerous compared to mountain climbing etc. but once did it in a classroom at a local college. I was working and stopped by to see a girlfreind who worked at the college. She had prepared herself that day by wearing stockings and garters, so there we went, with her on her back on a desk and me giving it to her. Her feet up in the air over my shoulders. Had anyone walked in on us we would have both been fired from our respective jobs, but it was very exhilarating.