Not a man (and I'm not sure why they'd have a more accurate answer anyway), but the way I understand this beyond mere self-reinforcing fetishism is that in a state of brutal nature male 'fitness' is self-evident.
That is to say, if a man claims you and is fucking you, that is in itself sufficient 'proof' that he deserves to be doing so.
It's like what other anons said about invaders. In the human evolutionary context that might be one group of humans attacking another, killing the men, and claiming the women. Sure, a woman has reasons to resist the initial attack, but once the men were killed she no longer has the means or even a reason to resist. She has to submit and her biology will tell her as much.
Now in a more civilised state a man can claim a woman by more socially constructed means (socially-accepted measures of accomplishment, wealth, grooming, a monogamy norm curbing female hypergamy etc...) and while in principle one could argue that's just the same thing by more sophisticated means, female biology is not that easily convinced. In the end the civilised world tends to spread the females more equitably rather than all of them being shared by a few alphas who buthered everyone else.
And I feel here lies the primary attraction of the barbaric orcish rapist. As he is outside the social order of a well-socialised, civilised woman, and has none of the merits that would award him a mate in her society, there can be no mistake that his rape of her is the primal kind that she is built to submit to. Further down the line this can all be brought to extremes and become ever more grotesque with porn-based reinforncement, but I feel this is the base of it.