Social contracts sketch social relationships in very broad terms. You can still have plenty of lifestyle diversity and plurality within them.
In fact you need a social contract to have any kind of diversity.
Otherwise a culture reliably degenerates into autocracy, which isn't known for its tolerance.
As for the diversity you speak of, I think it can be plausibly argued that many utopian conceptions of life really reduce to utilitarianism or hedonism. Diversity manifests in having different options for pleasure or utility. For a lot of people, that's inadequate.
I happen to be a philosophical liberal, and do not wish to live under a theocracy. Nonetheless, I think the fact that many of the highest aspirations of liberal philosophy amount to "having a good time" is a great risk that must be reckoned with, for it can undo the entire liberal project.
It is not religion but empathy that keeps people on the golden path- sociopaths gonna sociopath.