It may be useful to point out that in early drafts of the script for Alien Ripely was a male role.. Harleman sees the themes in the 1979 sci-fi/horror classic related to unwanted pregnancy, childbirth, and reads an abortive subtext in the various metaphors of the film.
But I think that O'Bannon's comments suggest that we can read Alien as an inversion of the viscitudes of the kinds of exploitation films that were common in the 1970s.
There are a few extra places we could go if we wanted to dive into allusions to Joseph Conrad but I'll spare you those details because you may be yet another person who doesn't like Joseph Conrad's work. So I'm not going to bore you as I have bored other people in real life by even discussing Conrad's work. :-) There, you can thank me now.
I recall reading some interesting comments from O'Bannon to the effect that he wanted to see men and not women sitting in theaters with their legs crossed and squirming at the stuff shown on the film. It's true the film can be read as a horror film about things related to childbirth, pregnancy, and the like but the motif of bodily violation is more prominent throughout the film. The 1979 sci-fi horror classic is many things but subtlety of subtext is not one of them.
It's true that horror films generally traffic in anxiety about sex but horror films also traffic in anxiety about how benign or (more usually) malevolent the universe we are in appears to be. Distilled down to its most material and materialist concern it could be summed up by Alan Moore's Walter Kovacs who says that it is not q god who foists all this miserable and pitiless cruelty and death upon us, it's us, and only us. Our mixture of frailty and cruelty is a horror that we can never truly get over or get around. Depending on who you ask the big mystery is not why there is evil in the human species but why there isn't more of it.