First, thanks for such a well-reasoned essay on a provocative, important subject.

In the end, we humans can't just leave everyone to live according to one's beliefs (though we should maximize in society the extent to which people can do that). The problem is the unavoidable necessity to govern the governance of society. Even "moral realists" are as completely subjective in that matter as any theist is because they inevitably 'rationally' (logically) "reason" from some subjective premise.

Perhaps the author of this essay--and anyone reading this Reply--might find interesting an approach to ethics that I have developed. It follows from the observation within (perceived) material existence that human beings--and the communities in which we live, given our inherent social nature to live together in groups--have no choice but to effect choices, i.e., choose among perceived alternatives and take action to bring that choice to fruition (which I got from Warren J. Samuels). That makes choosing integral to being human and respecting one another's capacity to choose the commonplace starting point for an ethic to govern relations among people in the context of a community of human beings: society. This ethic--respecting the capacity of others to choose for themselves--seamlessly applies to individuals as well as the structure and (sanctioned) functioning of the political process (the process of effecting choices for the community as whole) and the economy (the process of producing/acquiring goods/services — which is nothing but choices being effected).

If curious: "Can't Get Any Simpler" is a "2 min read" here in Medium with links to several articles about the ethic, none of it behind the paywall.