Here’s the theory I heard. Or maybe I’m just making it up. You be the judge:
Before mankind tamed fire to provide light at night, their only nighttime light sources were very feeble starlight, and the moon. A full moon meant a night well-lit enough to be safe from leopards and other predators that hunted in total darkness – and, thus, safe enough to make all those grunting noises and movements involved in carnal merry-making. Ovulation at or near the full moon thus ensured the greatest chances of impregnation, and thus a menstrual cycle or approximately one months’ duration that could synchronize with nighttime light-cues would result in the greatest reproductive success.
Nowadays, we light our night skies with thousand-watt sodium arclamps, chandeliers, etc., and so the light-cues of the lunar cycle are all but completely washed away. Thus, this month-long menstrual cycle has no “one special well-lit night” to synchronize itself with.
So, a night-light-swamped woman’s cycle has to fall back on the second-best cue it has at its disposal – namely, the pheremonal changes that accompany the menstrual cycle of the dominant female in her group. (At least one study has shown that it is the dominant female’s cycle to which the women in a close group will eventually synchronize.)
There are a few remnants of this old lunar dependency still lurking in our behavior, though. We still consider moonlight to be “romantic”. In poetry and art, the moon is usually considered “feminine”, the man-in-the-moon notwithstanding.
And of course, as we all know, a night light will scare away predators.
I’m not flying fast, just orbiting low.