The difference between ethical atheism and nihlism is that ethical atheists have more faith

[Neo-Scholastic]

That person should not vote, they should abandon all pretense of having a moral authority, they should not marry or raise a family because they will be unable to teach their children to be good people, they should not engage in any activity which requires moral reasoning, which is almost everything.

Sorry. I think you over-reached. IMO the better response is that you can do whatever you want as an atheist. You can chose to follow traditional moral codes or defy them, without any concern for consequences.The only restriction on behavior is a combination of pragmatism and self-preservation.

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Why not ...create a nationalistic morality? Why shouldn't one group enslave another group?

You are on point. If good and evil are arbitrary product of society then you cannot judge the relative merits of one nation's morality over another. You lose the right to say Anglo-American/French civil structure is morally superior to German National Socialism. If you are not resigned to "might makes right" then some form of higher moral standard, a highest Good, is required.

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I am not going to insult you, you aren't an honest person, and there is no reason to debate you.

Gentleness and grace, brother. They will call you all sorts of names, misrepresent our faith, and blasphem against God and His Word. Don't stoop to that level.