Didn't you know that there was an Alien-Ripley sex scene in the first movie? How do you think they found out she was pregnant in the third?
It was 15 minutes long, and you get to see a number of pleasurable uses for the Alien's probiscus. Audiences weren't ready for human-extraterrestrial love in the late '70s, though, so the scene was cut. You can download it on a few underground newsgroups.
