You could MAYBE point at Erotica as the first music video to really push the sexual content envelope.I was going to link "Dirrty" or something similar and say it explores the boundaries between (soft) porn and the video clip, which is important and whatnot, but that got me thinking; who or where did that trend start? Not "girls wearing short skirts" but really, obviously and deliberately, making a video as close as possible to softcore porn? My mind goes straight to Madonna, of course, but I'm not sure. Anyone?
Of course, now we have stuff like Rammstein's Pussy, which actually goes straight past soft-core and into hard-core. Money shots and all.