Ok, now were getting down to things I've only said 3 or 4 times. The function of morals is to promote the interest of the moral community. It is easy to figure out what would promote the interests of a given moral community by reasoning about it. You may have some leeway still to argue about what 'twould be best to promote, but it is still easy to see how reason can make good choices here.
Is the moral community arbitrary? Yes. It could be your city, your dance community, your genome, your species, your phyla, your planet, your church laity, your country, or the local gang of thugs you've fallen in with. Picking the best moral community to promote allegance to is also not a given, or an easy task, but it is still subject to the rules of reason, which can tell you that loyalty to some of these moral communities is pointless, and some is effective toward ends you and those around you pretty much universally agree are good. Is this a fallable process, an underminable process? Sure. Nobody promised you a moral rose garden where you didn't have to work and suffer setbacks for good ends. That is not an effective argument against it, either as to it's origins or it's effective results, since, abundantly on the historical record, exactly the same thing can be said, and demonstrated, regarding transcendental sources of absolute morality.
Then see my post 1080 and respond. Each person decides what is best for HIMSELF. Each person is a moral agent and does not make moral decision based on what is good for the community!