primate ancestry
    with carnivore modifications
    change to an upright posture
        bipedalism in trees
            orangutans
            bipedalism may be older than originally thought
        move from trees to savannah
humans use sex for more than procreation
    pair bonding
        sexual imprinting
    using sexual signals to re-motivate an aggressive male
    female continuously receptive - no estrus
naked skin
    increased sensitivity to touch
    blushing / sexual flush
        vaso-congestion
    complex facial expressions
face to face sex
    the vast majority of our visual signals are in the front
    eye dilation
        indicates interest
variations increase sexual novelty
    reduces sexual boredom in long bonded couples
female orgasm
    unique amongst primates
    keeps female horizontal
relatively large size of penis [for body weight]
importance of scent
    pheromones
        A chemical secreted by an animal... that influences the behavior or
            development of others of the same species [American Heritage Dictionary 3]
    flowery, oily & musky odors preferred at sexual maturity
sex as normal / enjoyable
sexual orientation
    heterosexual
    bisexual
    gay
    lesbian
    may not be gender identity [see LGBTQIA2S+]
consensual sex between adults
 
The Naked Ape - Desmond Morris
Why Sex Matters - Bobbi S. Low