Have to mention Julie Christie and Donald Sutherland in "Don't Look Now". It's long been rumoured that the infamous sex scene wasn't simulated. If you haven't seen "Don't Look Now" - it's a fantastic film, very creepy based upon a Daphne Dumaurier short story. First time I saw it, it scared the hell out of me.
Julie Christie is another actress who was super-hot (as well as a great actress). From memory - she had some standout sex scenes too (not too explicit though). Her under the table blow job for Warren Beatty in "Shampoo" is a very funny and memorable scene. She gets drugged, raped and impregnated by a rogue supercomputer in "the Demon Seed". She still looked great in "Away From Her" in her 60's.