I am not familiar with the issue, but to parrot other comments that are, it seems that because caste is not ethnicity or nationality, even having a law that recognizes caste as a difference os a step in the wrong direction. Making certain castes protected under the law has backfired horribly in India, and many do not want to see that repeated.

The nature of the law isn't relevant to the analogy. The point is that you can't say anything about the current state of the law simply by noting the size of the opposition to a new proposed law. The analogy would work just as well if it were a law that prohibited the killing of children, punishable by generational imprisonment. There are any number of reasons to oppose the introduction of such a law, and none of that would be evidence that killing children is currently legal.