No one really believes this. No one is out there saying that rape should be permissible, because "the government shouldn't legislate morality."
What the person making this argument usually really means is that in the area under discussion (typically something like sex, drugs, gambling, etc.) that person believes that government regulation should have little or no role. That is fine, and perhaps they are right. But make that argument, and not the silly one in the title of this post.