Alien is the... alien. Like the latin sounding word suggest, we imagine aliens as the ultimate strangers. And strangers cannot be like us, because there's a little xenophobic part in everyone of us that tells us that the more someone is similar to us, the more is safe. Aliens are depicted as slimy things because they're feared. Then, that's the scientific explaination, as earth people were evolved into a peculiar planet into peculiar conditions, some scientist thinks that even a little change in the surrounding could spell a completely different kind of humanity. For example, in a planet with a lower gravity, we could have longer, thinner individuals, in a planet with very thin atmosphere we could have a race of mutes with big eyes and skin like the chameleons, in a dark planet we could have blind aliens with echolocation and stuff. The fact is some scientific fiction of the '40 told us of aestethically pleasing aliens, mostly scantly clad lovely alien maidens searching for some brave dude to bring in space because they were short of good looking men. Then they would usually enslave and kill the unfortunate protagonist. It's more subtle... the fear of the people that seems equal to us, maybe even more friendly, but in fact they're just too strangers to mix

Well, considering we would judge anyone by human stardards, they would have to be pleasing humanoids to really take us aback. Excellent question.

Because not only is beauty inherited by humanity but so is our perception of it. We have evolved to see beauty in humanity, not necessarily because we are beautiful, but because "beauty is in the eyes of the beholder," and in all known cases the beholder is a human. We seek out humans to interact with and not aliens; for survival purposes. This is encoded in us... I kind of don't believe most of what I said. I believe we will always be the prettiest and that is what makes us so special. I believe that we are and will be the only species whose sexual attraction can be identified by many other species in mere seconds. I don't know if there is a super-pheremone or what, but I believe your dog can identify who the pretty ladies are, even though we can't tell who the sexiest cocker-spaniel is. But scientifically our tendencies have been reinforced to include a perception of 'pretty people.'