Is the Right Side of History right because it is right or because History says that it is right? If the former, then History is completely irrelevant in the determination of whatever its right side may be, and if the latter, then History is simply a whimsical tyrant. History may determine that today same-sex "marriage" is its right side and tomorrow homophobia is and, perhaps, next month Ethnic Cleansing. Right simply is at the mercy of Historical Happenstance.
Note to the lunatics: Replacing God with History does not escape the Euthyphro Dilemma.