Why We Call Bullshit:
What do aliens eat? Do the movies ever establish that? Well, they'd better eat the exact same damned diet as humans, since their offspring apparently have to survive on the exact same nutrients found in a human body. After all, a human embryo/fetus requires an incredibly specific set of nutrients, so much so that artificial wombs are still science fiction. And keep in mind, this species didn't evolve specifically to use humans as hosts--as far as we know, they had never encountered humans before the events of the first film.
Alien tries to bullshit its way around this by saying that the Alien's DNA merges with the host; this becomes totally ridiculous when you realize that, despite sharing a good 99.9 percent of our DNA with chimps, we're still genetically incompatible with the fuckers. Since the Alien is way, way less like us than the chimp, this would be like saying a Komodo Dragon could successfully impregnate a human.
But even that analogy breaks down when you realize that the Alien was not only completely different from humans, it was completely different from every living creature on the planet. Yes, unlike carbon-based life (a.k.a. f*****g everything), the Alien was a silicon-based creature: At one point in the movie, we see it replacing its exoskeleton with polarized silicon, which would be like if you replaced your skin with glass.