What is best for the cohesiveness of a society is also an "ought to be" because just like religious morality (as derived from societal mores), it is also not universally followed -- see the current subject of eugenics.
As there is no fact about the world that can vindicate the inescapable authority that moral judgments purport to have, you have, in terms of your own worldview no justification for endorsing ANY moral proposition at all.
Why not? It's what has been learned over thousands of years of civilization. Religious morality is built upon this rock, so can offer no more validity other than someone telling you a deity said it is so.
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.