If all he was after was ejaculation, then surely he could find some other means of 'relieving' himself. This is where I get stuck, because I am confused as to whether in fact a rapist has a distal reason for raping, or not. This is where I start to think that rape is a power thing, in which case, sex doesn't necessarily have to come into to it. Nevertheless, it does. Thus, after going around in circles, I think that a rapist is struggling to impose himself on others and dominate them, but can only really achieve this feat in his own mind by entering a woman's personal, intimate space.
Sex is the means by which to gain power.
So all in all, I agree with Stevan's comment that sex itself might be a power thing - rapists take it to the extreme.