SAM:
There are no atheist societies.
Except for the self defined ones, like theocracies; if they are not all atheist, its not for lack of trying. Really really hard. Ignoring the atrocities of atheists does not make them disappear.
Laws aren't based on things being unnatural. They are based on things being morally wrong.
Morally good does not equate to "natural" or "unnatural". That is the [enc]Appeal to nature[/enc] fallacy
Laws are based on ideas of right or wrong. Laws in all societies have risen from religion. If gay marriage is illegal, its because the religion in that society says so, not from any appeal to nature.