To be honest, I've been steel-manning the David as creator angle in this thread and others as it's an interesting discussion and I can see where John Logan et al. were driving at thematically. Which is good!
Personally, all David did was pour perversions of human sexuality into the primordial shoggoth-like pathogen; shoggoths are amorphous and can reflect a myriad of forms, in this case peeny and vagins. It stinks alright, it has the scent of f**k all over it.
As an explanation for the penis-y head and other references to human sex anatomy it works.
I know David as the creator or more accurately "sculptor" of the classical beast and life cycle is controversial among many fans, but it nonetheless remains consistent with Giger's aesthetic of perverse human sexuality and transfiguration; Giger's aesthetic wasn't so much predicated on "mystery" than perversion and transfiguration of human sexuality and machine. Genetically and by its nature the Alien is still Alien; its shoggoth-esque origins are primordial, the revelation is only that its phenotypic expression is a perversion and mockery of human sexuality/reproduction moulded by a sterile machine going mad.
He just gave an alien organism a more penis-y makeover.
Here's a great article that goes in depth analysing the Alien as a perversion of human sexuality - https://www.google.com/amp/s/plotandtheme.com/2016/05/18/the-xenomorph-and-the-perversion-of-sex-in-alien/amp/
"A completely foreign monster inspires fear, as we have no way of knowing what it can or will do. But, a monster which contains a kernel of our own humanity is far worse: it suggests not only that the creature may deprive us of life (and violently at that), but that it may deprive our soul of its cleanliness by reflecting our basest and most primal desires in its atavistic quest. These shades of humanity exist throughout the life of the Xenomorph, and are a critical component of its ability to dominate our nightmares with its disturbing and familiar sexuality.
The Xenomorph is a fundamentally sexual horror villain that exists on a biological continuum between the female and the male. Each aspect of its life cycle is punctuated by sexual aggression and forced penetration of its victim – ultimately culminating in a literal unprotected rape. And yet, each aspect of the Xenomorph contains a kernel of humanity which allows it to ascend from a monster-of-the-week to a primordial representation of human terror and insecurity."