The only one I thought of ahead of time - both the only one I thought of, and the one I thought of ahead of time - was Betty Blue (37.2 le Matin in v.o.) - which was amazing.
So was the peeling of the orange in that movie that wasn't Kentucky Fried Movie but was very like it. But since that wasn't actually sex - you just thought it was for the longest time - I guess technically it doesn't count.