Paul Hollander suggested that in the absence of an appeal to God the source of morality becomes politics. In a sense that is what the social contract argument is stating as well. The difficulty is that when it does so morality loses its universalist characteristic and becomes subjective as a function of political identification.
This isn't unique to secular sources for morality - it is the same difficulty that a non-universalist appeal to religion finds itself in when that is a function of a similar identification - the different rules with respect to believer versus non-believer in Islam are an example of this. For example, is it immoral to lie to another human being? Where are of the latter are equal in the sight of God, yes. Where they are differentiated by group identification as believer or kaffir, no.
I suggest therefore that the real issue is that in the absence of God no universal root of morality is possible, but that the mere acknowledgment of the presence of God does not guarantee it.
This has interesting echoes in Western legal theory (it is literally ALL of Islamic legal theory). Ask a professor of law sometime to explain the difference between malum prohibitum and malum in se and get ready for an earful regarding something called a "value consensus model." The roots of that consensus are precisely the roots of morality we're discussing here. Huge topic.
Cultures act exactly like a god offering benefits for conformity and penalties for nonconformity. The most obvious example of this is law and law enforcement, but stigma and exaltation work just as well or even better. Cultures exalt certain behaviors (think hip-hop culture rewarding narcisism or islamofascists blowing up innocent people being revered). The rapper with the most bling gets the most women so young urbanites want to be rappers and get bling.
Excellent post.