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On the one hand, sperm is physiologically cheap, extremely plentiful, and constantly replenished. Therefore, the more often and diversely a male spreads his sperm the more evolutionally successful he’ll be. Ova, on the other hand, are very precious, metabolically expensive, and can not be replenished (a woman is born with all the eggs she’ll ever have).
What’s more, women must assume the physical, emotional and metabolic demands of pregnancy. Hence, unlike most men who will gladly provide their sperm with little thought about it, evolution has shaped most women to be protective of their eggs and relatively discriminating of their sexual partners.
