Libertarianism is not Fascism. Fascism doesn’t mean “right wing politics I dislike”. It doesn’t mean patriarchy or sexism or even racism (which Heinlein was provably not). He was a radical individualist, and his ideology was absolutely at odds with ethnonationalism and authoritarianism, the two major hallmarks of fascism. He was progressive about a great many things including sexuality, religion, and individual freedoms. You can criticize him for being a capitalist, but don’t conflate it with fascism.