Without having experienced it, I doubt that you could comprehend it, but there can be a cathartic release in the "inability" to refuse. It is similar to the thrill one gets from roller coasters, base jumping or other high risk activities, or seeming high risk, even if you know there is a safety release should you feel it has gone too far.
Rape fantasies and forced sex fantasies are highly common, among both men and women, as both victim and assailant, the roles mixed.
45.8% of men in a 1980 study reported fantasizing during heterosexual
intercourse about "a scene where [they had] the impression of being raped
by a woman" (3.2% often and 42.6% sometimes), 44.7% of scenes where a seduced woman "pretends resisting" and 33% of raping a woman. - Crépault, Claude; Couture, Marcel (December 1980). "Men's erotic fantasies".
Archives of Sexual Behavior. Berlin, Germany:
Springer Science + Business Media.
9 (6): 565–81.
doi:
10.1007/BF01542159.
PMID 7458662.
When you have men and women who share such fantasies, you need a method by which to indicate that the scene needs to stop, and that the protests are real and not part of the scene. And they are usually for things not related to the fantasy. There are plenty of people out there that want to be able to say "no" during sex and other activities, and have it ignored. Hence the safe word.