While I am for relativism in the case of dealing aliens I am the very opposite in the case of us. Human rights are universal. When somebody tries to rationalize monarchy or genital mutilation with tradition he is simply wrong.
Of course the problem of such a stance is that it is on a fundamental level totalitarian. But you need this zero-level of dogmatism, you can e.g. not say that democracy is contingent and we can vote on whether we want it or not. In Germany this was possible eighty years ago and it led to fascism.
so you're saying you're a blatant speciesist? Ethics and rights apply to Humans, but not to sentient, self-aware aliens with their own societies?
What would be the distinction other than place of origin? If there were an undiscovered tribe living somewhere on Earth, would you contend that ethics don't apply to them?