That works if the process of gestation and giving birth is as straightforward as going for a haircut.
Unfortunately it's not. We have a nine-month gestation period, which uses up time that could otherwise be spent being fertile. Also, as a species, our offspring spend several years being dependent on their parents for food and shelter. In this case, it is therefore beneficial to the survival of the offspring if the father sticks around to help provide food (as opposed to Mum going out to forage and leaving the children vulnerable). However, this is also an incentive not to get knocked up by a different partner in the meantime - Dad is less likely to stick around to raise the kids if he can't be sure they're his own. This therefore limits Mum's options, so it makes sense to be more choosy about the partner if it might be her only one.
(LongJohnBaldry, Mon 17 Jan 2011, 16:42, Reply)