Fist, take a look at this and then I’ll respond and I invite you to do so also.
OK. Good points, articulated with reason. I wouldn't expect a tough challenge to be presented from a theistic argument. It doesn’t anger me to have to deal with objections to my world-view raised in this manner. But deal with them, I shall.
I think it is quite possible that there are advantages to having a set of moral imperatives that everyone in society is expected to rally around. I can certainly see how, in the evolutionary environment, the laws "given by gods” and enforced by tribal leaders, obeyed and revered by members, led to a cohesiveness that would have been otherwise impossible.
But there are many things that we have evolved to “do” or “be” that we now discard. Other things we modify. But we are nothing special minus our intellect, with our ability to contemplate and conceptualize.
To say that we don’t accept many of our old gut-level instincts (usually reinforced by sacred beliefs) is not to say that we don’t have a set of values that we tentatively treat as absolute, even if we are now bright enough to know there is no such thing. As recently as 200 years ago, the Founders, acting within the higher principles of enlightenment, created a society that treated females as second-class citizens (based on the old morals). Happily, we didn’t hold that as an absolute. Many other moral codes are falling or changing.
If some members of our developing society become overwhelmed and desperate because the old “objective” (read: god given) morals are being pushed aside for new “subjective” (read: arrived at, tentatively, by reason) then those people must necessarily fall by the wayside. It’s either that, or a society, clinging to “objective” morals will defeat the forces of humanism and we will have a rebirth of the Dark Ages. A highly cohesive society, running under strict, god-given principles is an enemy of secularism that will not easily be defeated. They do have an advantage in unity of thought that we will never have.