the genital eye

Sex amongst people is an indoor act done with the lights down low, or turned off.  Not even our sleep is as consistently so: siestas in the hammock can be in the bright afternoon, even if in the shade.  Why is this?  It is not my impression that animals seek out dark or enclosure for sex.  Nor is our sexuality, as distinct from our sexual activity, a darkened-room thing: nakedness, pornography, erotic poses - whether dominated by flesh or clothes - are generally well lit.  It is something to do with clothes.