Good can be taken to be what is best for the cohesiveness of a society only if it is antecedently the case that cohesiveness of a society is itself "good." You are still assuming what you must prove.
The reason that a naturalistic worldview offers no justification for endorsing ANY moral proposition at all is that you have to commit the naturalistic fallacy, or Hume's rule against deriving an "ought" from an "is" to do so. Reason, logic, and rationality can tell you what to do to achieve a particular end, but it cannot tell you what end you ought to achieve in the first place.
Quite right you are.
You have pointed out a fallacy that seems to escape the humanist/naturalist who denies the existence of absolute morality.
When it finally dawns on many of them they simply redouble their vigor, though I have encountered a few honest exceptions.