I can't comment on what the RPG book/ expanded media is pushing, but going by the movie alone (and taking into account the time period the movie was made and its Vietnam war influences) to me personally Arcturia is just another human colony. The marines' crass comments regarding "Arcturian poontang" and "the one you had was a man" was typical non-PC /ignorant 'lad banter' that you may experience from those on tour in places such as Thailand / east Asia, where Kathoey (or 'third gender' as it is sometimes referred) is embraced - I doubt such poor-taste comments would have been allowed in the movie had it been made these days, it is a product of its time. But nothing, not one thing in the movie ever referred to Arcturians being alien. I mean, you could also read it a totally different way, taking into account the alien universe; they could be synthetic sex workers. The marine is justifying that the gender was irrelevant as it was just a machine, and not a real guy (sort of a "it's not gay if it's plastic" kind of comment) there's so many ways you *could* read into that, but absolutely zero evidence that extraterrestrials are known to humankind (which is why the Jockey and Xenomorph are such a big deal).
