LOL. I admit, I certainly didn't see this tack coming :)
Okay, so concerned are we with the notion that moral systems might all be subjective in some sense, that we merely redefine "objective" in such a way as to cover all cases. So, no matter how we define the actual rules of baseball, they're always "objective" - if the new rule in baseball allows tackling on the basepaths, that's an "objective" baseball rule. And naturally, no matter how we define the laws of society, they're "objective" as well by the same exact logic. Whatever rules we construct for a moral system, that moral system is "objective" by virtue of the fact that all the rules are "objective". So a moral system with the rule that infanticide is permissible is still an "objective" moral system.
My, my. That's a rather clever solution to the problem of subjectivity - just define everything as objective. Of course, "objectivity" loses all value for differentiating between moral systems, in that case - all moral systems are equally "objective", and hence none can claim to be somehow "better" than any other.
Morality, like the rules of baseball, requires an authoritative voice because moral rules come in the form of personal, propositional commands from an authoritative source.
Of course, as in the case of baseball, that "authoritative source" can be composed of human beings. The commissioner of baseball, despite certain delusions otherwise, is neither omniscient nor omnipotent, and yet the rules committee under his purview has created an "objective" set of rules. Similarly, humans can just as easily create "objective" rules that govern non-baseball behavior. In which case, atheism may as well be the de facto approach - the commish is no less authoritative by virtue of being an atheist, and neither would anyone else be.
If everyone makes up their own rules which are all equally legitimate the center fielder has no grounds for complaint...
See, the problem you're having here is that nobody is saying that it's either God or no rules at all. Actually, I should say that you're trying to say that, but not very successfully.
The question is whether moral systems are possible without invoking a Creator. Of course they are. All that's required is that we agree on what the rules are, and agree to abide by them, just like in baseball. If you and I are neighbors, and we agree to allow each other to borrow tools from our respective garages and return them without needing to seek permission first, we have created a moral rule to be in effect between us. We didn't need God to tell us that's how we should behave, we didn't need some external third party to tell us that's how we should behave - we agreed on the acceptability of some behavior, and then behaved accordingly. This does not mean that you have "no grounds for complaint" if this rule we have agreed upon is violated - if I take your drill and then keep it for myself, for example. Extend that to society at large, and presto - a moral system by consensus, with the newly discovered virtue of being an "objective" moral system.
...which precisely illustrates my point that if you make a moral objection to something you are assuming some moral standard which the thing violates...
Of course, but that in no way obviates atheists as moral actors. The whole point is that you can create coherent moral systems without appealing to some other thing. If we agree that it's immoral to fail to tip your cap when a lady walks by, then we've created a moral standard for ourselves. An "objective" one, no less.
I've been thinking about your example of a moral system with the rule that infanticide is permissible. It seems to me that such a "moral system", if such were possible, illustrates why conventionalist, or a moral system by consensus, defining what is right does not really encompass what morality is, and why moral rules are not arbitrary like the rules of baseball.
Aside from the ambiguities of what constitutes "society" (which "society" does one obey?), if society is both the origin and justification of morality, several counterintuitive examples come to mind:
1.If two people lived on an island and shared no society between them one could go over and kill the other on a lark and no moral rule would be violated.
2. If there is no moral law above society, no external standard, there is no basis to criticize or oppose ANY other societies' practices, no matter how repugnant to outsiders those practices might be. Stuffing Jewish people into gas chambers is ok if it's done by consensus.
3. If society is the origin and final measure of morality then all its laws are moral by definition; there can be no such thing as an unjust law.
4. If morality is defined by present society's standard, then any challenge to that standard by a moral reformer would by definition be acting immorally, which seems oxymoronic.
Cordially,