It is not unexpected to want and choose to believe your partner is/was in the right, to feel a duty and desire to support them, in which case any blame is automatically put on the other party.
In a culture where one isn't even supposed to show much of oneself in public, and the media shows more extreme behaviour of individuals here, it is little wonder the idea that those in western society are acting poorly and at fault becomes common; but the real issue is a lack of freedom on the part of the other culture. All cultures have things to learn and try to put right.
The reality isn't how the partner to the convicted perceive it, but that fact's not helping.
A culture has to modernise at it's own pace and make the same mistakes as cultures that changed before them, made. Blaming the victim is something that will go in time.
But one can not expect a host country to tolerate everything the incoming citizens were used to in their original culture. Keeping a flavour of home is fair enough, it can add to the host country's experience in a good way. Broaden understanding, experience, horizons. But those who come need to make a change as individuals, to accept and fit in with where they decide they would prefer to be, and leave their homeland to make a more gradual evolution in its own time.
As for how many successful prosecutions for 'marital rape' there have been. as for every other crime, a case needs to be brought and proven. I'd suspect the number reflects the occasions where that has occurred.