Remember how when you were around 16 or 17 you figured out that Alien is chock-full of phallic and yonic imagery and when the facehuggers plant their baby alien in you they're really humping your face? Then you give birth to its baby–out of your chest. Never came to this realization? Well let me help you out with some cold hard facts. Here's a quote from original Alien screenwriter Dan O'Bannon:
I am going to put in every image I can think of to make the men in the audience cross their legs. Homosexual oral rape, birth. The thing lays its eggs down your throat, the whole number.
Yeah, well, you remember when you figured that out and you thought to yourself "Heh, that's no big deal, it makes everything seem a little grosser but in place of traditional sci-fi I now have a complex layered metaphor designed to make ignorant men confront their ignorance of a disgusting real-world issue"?
Well, it turns out the Brits also figured it out, but the logical conclusion that you came to when you understood the complexities of Alien was not the one that the BBFC (British Board of Film Classification) came to. They're job is to give the movie a classification (they're essentially the MPAA of the U.K.) and deem what age group the movie is fit for.
Well as you probably guessed: the BBFC slapped an X rating on the movie. Back then, the British X was similar to what was an X in the US and is now equivalent to the current NC-17 rating) thanks to its disturbing content. It wasn't the copious amounts of blood and violence on display in the movie. It wasn't even really because of the whole metaphor for rape thing... it was WEIRDER!
According to Blastr the BBFC felt that the visuals of the Alien impregnating John Hurt's intrepid explorer would "confuse teenagers" as to how proper sexual conduct is observed.
In short: They thought Alien would make teenagers hump each other's mouths.
One censor said:
I don’t want to flash ideas like this to teenagers who might not have come to terms with the normal sexual functions… The early teens are a troublesome time with physical changes making terrific demands on emotional stability. I don’t myself want to pass for this age-group a film which might be disturbing in a non-specific way to a significant proportion of them.
A slightly smarter but no less paranoid censor said:
I feel uneasy about passing for 14-year-olds a film which uses sexual imagery in a horror context… The images are not always explicit but run like a dark undercurrent throughout suggesting a powerful, threatening, unnamed force. Occasionally the image is explicit as when the leathery egg opens up to reveal a glistening pulsating membrane which erupts into a squid-like creature.
I feel uneasy knowing that this nervous square was in charge of what my fictional British parents were allowed to see at the movies. That description alone makes it sound much worse in way I never imagined. Thanks, Mr. (or Ms.) Censor Person. Before it was just an alien attacking someone's face, and now 'pulsating membranes' are all that I can think about.