My Years With Proust - Day 394

   But a day or two later, I received proof of this young woman's tendencies, and also of the probability of her having known Albertine in the past.  Often, in the hall of the Casino, when two girls were smitten with mutual desire, a sort of luminous phenomenon occurred, as it were a phosphorescent trail flashing from one to the other.  It may be noted, incidentally, that it is by the aid of such materialisations, impalpable though they be, by these astral signs that set a whole section of the atmosphere ablaze, that dispersed Gomorrah strives, in every town in every village, to reunite its separated members, to reconstruct the biblical city while everything the same efforts are being made, be it in view of but a momentary reconstruction, by the nostalgic, the hypocritical, sometimes the courageous exiles of Sodom.

Marcel Proust, Cities of the Plain, pp. 880-881

Proust writes of that in the case of "mutual desire, a sort of luminous phenomenon occurred, as it were a phosphorescent trail flashing from one to the other."  In this case he is speaking of two women, but it seems to be the case with every couple who are at the peak of love/lust, at least if you're doing it right.  A couple different times in my life I can remember being involved with a woman wherein we were trying to keep on the down low and were quite certain that no one knew anything, except that everyone knew exactly what was going on, because we were emitting that "phosphorescent trail flashing from one to the other."  There probably is some sort of evolutionary logic to it, as it, allegedly, scares off lurking predators.