There is literally no moment in Alien in which Ellen Ripley (Sigourney Weaver) would form a love connection. In earlier drafts of the script there was a bit of sexytime between her and Dallas (Tom Skerritt), but director Ridley Scott didn’t film it, reportedly because he decided it would throw off the flow of the film. I read that it would have taken place right after the chestburster scene, so I have to say: Good call, Ridley.
Plus, originally all the Nostromo crew members were given gender-neutral names, the idea being that they could be cast as either gender. The writers put some thought while they were developing the script into how sex would work on these long-distance space flights, namely the fact that it wouldn’t involve romance at all. It would just be a form of stress relief. Basically, the people behind Alien could not have given less of a flip about romance if they’d tried.
Killing the alien is kind of the priority, after all.