Unless you willingly stifle it, the male body spontaneous response to sexual stimulation includes a cadenced contraction of the abs and rear thigh muscles, generating the typical fucking motion. This specific muscular reaction ensures the male’s arousal will be nurtured and fostered without requiring any conscious work whatsoever. As the male’s arousal strengthens, he can abandon himself knowing very well that his body will make sure his penis gets all the exposure it needs to trigger off the orgasm. Nature, of course, doesn’t care about the male’s orgasm : the final goal is to make certain the male will deliver his sperm easily when required. Making the fucking motion as spontaneous and mechanical as possible is part of that master plan.