Saucer Girls Have Big Hips.
A logical hypothesis based on three established concepts:
1. What we know about UFO alien life-forms.
2. What we know about parallel evolution.
3. What we know about paleo-gynecology.

1. Alien Life-Forms.
There are enough creditable eye witness reports of encounters with UFOs and their occupants to establish the basic alien physiology, I.E. their physical attributes. They are of course bipedal, (they walk on two legs), they have a torso, two arms, a neck, and a head with eyes, a mouth, (ears, and nose Optional) . The head is a central point of this thesis, in that, in almost all reports of encounters with aliens, they are described as having very large heads.

The head is much larger in relation to the body size than that of human beings. This fact is only logical since any creature intelligent enough for inter-stellar travel, must have larger brains than we do. It is common knowledge that the ratio of brain weight to body weight * is a guide to intellectual capacity.

* Refers to the species average and not individuals.

2. Parallel Evolution.
Parallel evolution occurs when two species without a direct genetic link evolve similar physical characteristics when they exist in similar environmental niches.
Examples of this are:
A. The Wolf and the Marsupial Wolf, two animals that, though genetically distinct from one another have similar physical characteristics because both species have adapted to similar environments, and ways of life.
B. The modern day Porpoise, and the Jurassic Ichthyosaur; two terrestrial living animals that had evolved into totally marine living animals. Both doing so by exploiting the same environmental niche, and both developing similar physical characteristics, and appearances.

The basic similarities of humans and UFO Aliens indicate that parallel evolution has been occurring , bringing both ourselves and the aliens into similar body designs. Just as we did, they must have evolved from four legged animals. At some point in their line these animals adapted to arboreal living, thus developing attributes such as binocular vision, and dexterity in the forward limbs, both being required for tree living. This is the path our own genetic line took in order for us to evolve with these traits. Then at some time readapting to life on the ground, but evolving into a bipedal form, and thus freeing the forward limbs for what would have been the very first tool use, (picking up a rock and throwing it). Tool use was the strongest form of evolutionary pressure that allowed the human brain to develop far beyond our animal ancestry. Just as it must have been for the UFO Aliens.

3. Paleo-Gynecology
In the 1970's the first fossil hominid specimen of Australopithecus afarensis (southern ape of the afar) was discovered in the Afar Triangle of North-East Africa. The most remarkable item of this 3 to 4 million year old skeleton was the mostly complete pelvis, for it is much closer to the modern human pelvis in shape and function than modern or prehistory Apes. The pelvis shape in combination with the knee bones indicated that this animal walked fully erect. The brain case, however, was only slightly larger than modern Chimpanzees. The main point for this text is that the birth canal in the pelvis of this animal was small in relation to the human female birth canal. Because the animal's brain was much smaller than humans, the birth canal was in proportion to that physical characteristic. Where as, with the modern human's greater brain size, there is much greater brain development in the womb and the birth canal is larger in-order to pass the larger brained infant. It is this biological requirement (larger birth canal, i.e. "a wider pelvis" as compared to the male.), that gives the human female that wonderful "Aspect Ratio" or ("Hour Glass Figure") that the human 'Male' so admires.
The "Hour Glass Figure" is something that the early Australopithecus females did not have, due to the smaller birth canal.

little wonder they died out.

So applying this brain size versus birth canal size requirement to a alien humanoid form with even greater brain size, one must conclude that the birth canal for the females of the UFO alien life-form must without doubt be much larger than their human females counterparts, in order to pass the larger brained infant.
The larger birth canal would require a wider pelvis. so these alien girls must truly have hips from hell.
And this gives us a more reasonable explanation of why the UFO aliens are here on this Earth. Obviously girls from any other planet that have really, really wide hips would be less than attractive, especially when compared to the females of our own Human species .
(lets face it, Earth girls are the best)
So, it seems obvious that the UFO aliens (the Males at least) are here for one over-riding and understandable reason..

They want our women.