He can still have an erection. In general, castrated men experience a much-diminished sex drive, because their bodies have very low levels of the male hormone testosterone. This lowers the frequency, strength, and duration of erections, and can cause hot flashes, vertigo, loss of body hair, and breast growth. But depending on the individual, it may be possible for him to become aroused and even to ejaculate, although his erection may be modest and there won't be any sperm in his semen. Even if a castrated man can't maintain an erection, he can temporarily reverse the effect by taking testosterone. Also, rapists aren't necessarily driven by sexual desire; a lower sex drive won't prevent attacks that are motivated by a desire for power.