Its testosterone, of course. A paper covered in New Scientist found reduced levels in fathers and in older Senegalese men and higher levels in men with multiple wives. The conclusion was that fatherhood lowers levels. There's an interesting circle of casue and effect here that they might have got the wrong way round. Testosterone is associated with dominance in men (pdf), as anyone who has been threatened with a slap by a two metre wide bouncer knows. Could it be that the domestic servitude of diligent and conscientious fatherhood stifles our inherent manliness in a manner akin to the subordinate life of an omega-male and quenching release of the man-essence? (No sperm-related puns, please).
Addition 02-09-09:
Female gorillas imitate oestrus and engage in copulation when pregnant in order to obstruct fertile competitors mating with them.
Addition 02-09-09:
Female gorillas imitate oestrus and engage in copulation when pregnant in order to obstruct fertile competitors mating with them.