valkyrie:
- why do more sexually-frustrated women not commit rapes?
Probably because women have lower libidos and less testosterone than men, are generally physically weaker than men, have different biological needs when it comes to sex, and can usually get consensual sex from a random stranger if that's what they really want.
- why do some [male] rapists deliberately target females who are past their prime [with respect to aspects of fertility and the biological 'reproductive' urge]
Very few do. The vast majority of rape victims are young, fertile women. (See http://66.216.123.69/RTC/Facts+and+Quotes/Statistics/) There is even some evidence to suggest that rape results in a higher incidence of pregnancy than does consensual sex. (See http://news.bbc.co.uk/2/hi/health/1398894.stm)
- why are many rapists unable to 'perform' in consensual sexual situations, but are provoked to sexual excitement at the thought/prosprect of forced intercourse?
Possibly because those men who actually find rape more exciting than consensual sex are more likely to commit the act. Do you have any statistics for the percentage of rapists in this category?