Though there are some primates that do not advertise themselves as sexually available only when in estrus: in humans, sex does not have to be for reproductive reasons (as proven by the emergence of contraceptives) Sex for fun: animals do not have the concept of marriage. If both parents are feeding a helpless baby, then the chances of its survival are very high: only the mother could take care of the offspring. In this case, the father leaves to go off and copulate with another female. Females often invest more in the offspring, hence they stay to protect, feed and tech the offspring until it is capable of surviving on its own: only the father could take care of the offspring. This is why in the majority of animal species, the male does not help in the raising of offspring: if the male decides to stick around, he needs to be certain in his paternity.