Alternatively, they were only white males because the actors hired were hired for sexist and racist reasons and it wasn't any thematic portrayal.
Besides which, that's not true. The colonist in Aliens walled up in the nest, the second time we ever see an alien hatching from a host (ignoring Weaver's dream sequence), was female. So from my memory, you had the guy on the first ship, and only one was impregnated with 2 females and 1 black guy where chance on the first victim would favour a white, make crew-member, so there wasn't anything thematic there - it's not like all the white guys got impregnated and the woman and black dude died otherwise. In Aliens, you had everyone in the nest impregnated by implication, and the one shown hatching was female. So no theme there. I don't recall the other films but given movies are typically predominantly populated by white blokes, even more so as you go back the 70s and 80s, it stands to reason that everything bad that happens is against white blokes.
In short, I disagree with your idea of the theme. I think the Aliens were unprejudiced in their murder and mayhem.