I'd like to hear a theory about why this might be evolutionarily true. Was it because the men were out in the sun all day, decreasing the contrast of their skin, while women working together tend to have more contrast because they likely lived and worked in the shade?
I wouldn't think evolution has anything to do with it. Quite simply, the face with lower contrast is perceived to be flatter. (The reason is purely geometric -- a curvier surface contains a wider range of angles-of-incidence with respect to the lighting source, thus resulting in a higher-contrast image.)
By placing the two images next to each other, our brain assumes that they are illuminated by the same lighting source, immediately tags one as "round" and one as "flat". Women, by the very nature of having "higher cheekbones" (and typically more facial fat), have rounder cheeks. Thus, if the brain tries to tag the two images as different sexes, it's much more likely to tag the higher-contrast (rounder-looking) face as female and the lower-contrast (flatter-looking) face as male.
Helping (hurting?) the situation is also the fact that the black level of the eyebrows was not kept constant when adjusting the contrast, thus causing them to lighten and look thinner in the higher-contrast image. Women tend to have thinner eyebrows (at least in modern society, due to the fashion trend of eyebrow plucking), no doubt lending weight to the brain's decision.
As to why women have higher cheekbones and more facial fat, well that question I'll have to defer to an evolutionary biologist :)
Men have darker skin because of higher levels of hemoglobin (partly due to not bleeding every month). Maybe that makes the skin in the face more ruddy, decreasing contrast with the lips and eyes.
I remember reading that both sexes prefer less hair, but that testosterone keeps men hairy. Probably googling for a recent "naked ape" article would get it for you.
If women worked in the fields tending crops while men hunted animals (in wooded areas), it could actually work out the other way around in terms of exposure.
In any event, its not clear that phenomena can have an evolutionary basis. Regardless of the perception of sexual gender in a potential mate, if the other is in fact of the same sex, there is no procreation, so its not clear how the selection mechanism would be operative. There are (to be polite) far more suggestive indicators of sexuality in the human body which are fairly readily apparent and far less prone to miscommunication.
