Following on the all too often socially condoned 'when is a crime not a crime' trope ...
A. When the person raped is of a low caste or class.
Rape is often seen primarily not as a crime against the person but as a crime against property. It has been so in England. Even today upper class and whiter women in UK are cared about more than lower class and darker women. Poor women are seen as having no honour and called names like 'chav'. Poor women's sexuality is seen as coarser, less refined than upper class women's, therefore rape of poor women does not count as rape.
Here's Bojja Tarakam, senior advocate, activist based in Andhra Pradesh commenting on the recent furore over one gang-rape in Delhi. Video from DalitCamera.
And also on DalitCamera, Sujatha Surepally, working on land and anti people's development models, environment, human rights and women's rights. Video.
Rape can be seen as a useful tool for controlling the lower orders if they show signs of getting 'uppity'. Perhaps most famously to us now in the story of Phoolan Devi (see Vidya Bushan Rawat's August 2012 interview with PD's sister, explaining how she has been dumped back in poverty by the Samajwadi Party, here if ye ken Hindi). Has also been used historically in a huge variety of permutations, as well as against men, as example here.