I have previously posted the following conduct guides
1. If it harms no one, do it.
2. Protect the weak from the strong.
Obviously the extermination camp example runs afoul of the above. If a law violates one of the above, it should be opposed. Gay marriage does not.
I am not for gay marriage. I think SCOTUS made a mistake in the breadth of the ruling and should have left it to the individual states. They made the same mistake with Roe. Obviously, on this board, I am in the minority. Time will tell which path would have been better.