I agree in part. That's why morals are so arbitrary. Look around the world and see the differences. However, the shared morals are the basics that allow us to live and work together - not arbitrary at all. For me, what appears to be arbitrary is the choice of the end result you want. The kind of society you are trying to build. You alter the moral code to promote those goals. Want men to dominate? Make it okay to beat your woman with an appropriately-sized stick. It's okay with me if someone chooses a set of rules that somebody else figured out (religion, laws) since that job is too big for any one person.
That said, let me suggest this for atheists (and secular society): Decide the end goals you want (rights, freedoms, protections, responsibilities) then use Game Theory to come up with a suitable rule set with our current mores as a starting point. We will be able to derive a landscape of livable societies that come close to meeting those goals. Soon, we'll have enough computing power to model and simulate whole societies and we will be able to figure out how to proceed.