In neurological or biochemical terms, the after sex effect or refractory period could be linked to the release of oxytocin (one of the peptide hormones) in the pituitary gland (in the center of the brain).
http://en.wikipedia.org/wiki/Oxytocin
http://en.wikipedia.org/wiki/Refractory_period_(sex)
Another possible cause for the refractory period is the release of prolactin, which represses dopamine (dopamine is responsible for sexual arousal).
Prolactin is another peptide hormone
