Sexual symbolism and reproduction are present throughout the film.
Kane's attack by the facehugger and later giving birth to its offspring, the Alien, is an obvious example.
The facehugger penetrates and inseminates Kane, symbolic of human reproduction.
Ridley Scott was explicitly interested in using sexual imagery in Alien, he claimed it was meant to represent the idea of fearing one's offspring, and the horrific side of reproduction.
When the Alien dies, it's only an adolescent or young adult, the 'nightmare offspring', a literalized representation of the fear that a child will grow into something horrible.
The entrance to the alien spaceship can also be interpreted as a vaginal opening, with its interior being womb-like and filled with eggs. In this respect the alien spaceship is representative of a womb, of the basis of life.
The main computer is also called 'Mother' which is a symbol for maternal care. It's programmed to take care of the Alien, symbolizing the archetypal mother protecting her offspring.
Another example of sexual imagery is when Ash tries to kill Ripley.
Ash shoves a rolled-upmagazine that appears to be pornographic in Ripley's throat in an attempt to suffocate her.
The magazine becomes a very phallic object, and the forcing of it down her throat comes across as a forced act of sexual violence.
The Alien's method of reproduction through implanting the embryo in a host leads to a violent birth, it blurs the lines between creation vs destruction and life vs death.
The chest burster scene confronts the audience with the fears associated with bodily invasion and loss of bodily integrity (self-ownership of your body).
The design of the Alien itself is particularly phallic, with its elongated head shape, and subtly hints notions of sexual threat and violence.
The Alien's method of reproduction (the facehugger) is violent. Its attack is invasive and personal- the way it forcibly implants an embryo into its hose could be seen as a metaphor for sexualassault.
In the final scenes of the film, Ripley is sexualised both vulnerably and empoweringly.
She is in her 'space underwear' which shows her vulnerability, yet her character remains strong and capable subverting typical genderroles.
Some have linked the face-hugger's attack on Kane as a form of male rape, again showing the inversion of traditional gender roles in horror scenes.
The facehugger forcefully implants an embryo into a male host, this presents an incredibly disturbing image of violation and impregnation, traditionally associated with femininity.
O'Bannon, the film's screenwriter, said that one of his intentions was to represent male fears of penetration and impregnation.
Usually female characters are the victims of sexualised violence- 'Alien' inverts this, showing male characters can also be vulnerable to these forms of violation.
The Alien's life cycle represents non-consensual reproduction.
The first stage, the facehugger, represents forced impregnation, and the last stage, the chest-burster scene, represents forced birth. The facehugger initiates a process that's invasive and violative.
The chest-burster scene reimagines birth as a violent and destructive act.
It subverts the natural association of birth with creation and joy, instead presenting it as a moment of death and horror.