"Civil rights should be grounded in our existential plight of being relatively free, intelligent mammals struggling to find meaning in a meaningless natural order."

That would be nice, but there is at least one stripe of atheism that is anti-metaphysical tout court. Some atheists believe that civil rights are "grounded" in the plus-sum outcome that emerges from respecting them, end of story. Everything else is in its own way a form of theology.

Some people may ponder why we are here, but that's on them. As the bus-riders' song goes, we're here because we're here. Now let's get on with making here the best place we can make it for ourselves, which, thinking statistically, means making it the best place we can make it for the most people and hoping we are in that majority.