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.