> If Prop8 had won, would it have been on the right side of history?
Prop 8 did win. I don't know if it is still on the books; it was rendered inoperative due to a later Supreme Court decision based, so far as one can tell, on neither the text nor the intent of the Constitution. In other words, whether one agrees with the decision or not, it was unconstitutional.
Right, fair point, but I mean, the "wrong side of history" narrative tends to presume that either in the long run, whatever side is right will win, or that whichever side wins will be the right one. I think that's a very dangerous assumption either way that should really not be made into the core of a moral argument. Argue from universal principles, argue from universal emotions, argue from social health, argue from self-interest. But don't argue from your eventual presumed victory as if that's a reason in itself.
What does that even mean? If Prop8 had won, would it have been on the right side of history?