men need to be willing to have sex to make it happen.

erection /=/ willing to have sex.

Biologically, it's simply a response to a (physical and/or mental) stimulus. In the case of rape, the mental stimulation is absent, since the victim is not attracted to their rapist. However, the physical stimulus is there. A lot of women orgasm when being raped, that doesn't mean they were "willing" to have sex, it's a physiological response to external factors, in this case, penetration. In the case of a (gay) man, if a (naked) woman started groping him, touching his crotch and forcing herself onto him, he'd eventually get an erection, that doesn't mean he was attracted to her, or willing to have sex, she just induced the boner by touching/kissing/teasing him.