I wonder if it has to do with mammals being warm blooded. Reptiles use internal fertilization but have a much lower body temperature. It doesn't explain about birds, unless it's an evolutionary divergance.
I would imagine a knee to the groin would be painful to either sex, considering how many nerve endings are there. The diagram on the right details the proportion of brain tissue that processes sensory information from each body part. This also explains why a paper cut is so painful, since you damage a lot of nerve endings that are close together.
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