Evolution is opportunistic, and builds on parts it already has.
The stickleback fish can easily be flipped from courtship to attack, for example. But, according to sociobiology and inclusive fitness considerations, since those were the conditions prevailing in our EEA, it would be suprising if they did not continue to make themselves felt proximally today. Probably there's not much point in trying to second-guess the motives of the rapist. To a first approximation, what he gets is presumably what he wants. The distal reason for that attraction and pleasure is reproduction.