Congratulations, you just made a moral claim: "we ought not to do what is detrimental to society". Now let's examine that claim. Why ought we not do this? Is societal unity an intrinsic good to be sought? Arguably... but probably not. Don't we value societal unity for other underlying reasons? Promoting the happiness and wellbeing of the poeple, for example? For the sake of argument, let's say "No, societal unity is a good in and of itself, not a means to some other end." Now, let's take the idea of societal unity to its logical extreme and see if we can come up with a reductio. Imagine a thoroughly unified society, say, the society in Huxley's Brave New World. Imagine that this society had no dissidents. Would that really be preferable to a somewhat disjointed society (say, the one we have now, with its occasional shoplifting and theft), would you really want to live there? Probably not... that's why we call it a dystopia. It's also patently undemocratic, since dissent is hugely important to democratic government. Take another example: is support of a thoroughly unified Nazi German state preferable to supporting a state that allows for some disunity? These are intended to be extreme counterexamples that your basic principle will allow. I don't think societal unity is the primary good we should be after in establishing morality. Fortunately, every relevant philosopher I've ever heard of agrees with me.
You might say, "hey, I never said UNITY was important. I had in mind another conception of the words, "good for society". Well, what conception? The prospering of society? Surely economic gain can't be an operating fundamental principle; economic gain is prima facie an instrumental good, not an inherent one; we want economic gain to improve the quality of our lives, to increase our collective happiness. Well, what about the happiness of people in society? Now you're into utilitarianism, which has its own problems... do we tally the happiness of everyone up somehow (in which case it seems that making babies is moral in so far as it contributes a new source of happiness, which is a very odd statement)? Or do we rather take the average happiness of everyone, in which case, wouldn't it help to just euthanize all the unhappy people, starting with, say, the homeless?
If you're arguing that morality is derived from "principles in a society that are regarded as commonly acceptable behaviour", to use your exact words, you're a relativist. You're effectively saying that "whatever a society agrees should be moral, is moral for that society". This exposes you to another nazi counterexample: their society determined that the execution of Jews was moral. Should that imply that this practice was, for them, a morally justifiable one? Most would say no. Your statements do not allow for ANY moral criticism of other societies, and as a result may be inherently self-defeating because as I've already said, it's extremely difficult to define a society; the boundaries are obviously vague. What's to stop me from going into the bush with five hundred followers to form a society where human sacrifice is moral? How about fifty followers? How about three? Relativism of the kind you imply reduces to individualism (whatever I think is moral for me, is moral for me), which is, by the way, the closest theory to skepticism. I believe the word that best applies to this development of your argument, therefore, is "oops".
(PS relativism also completely negates the possibility of moral progress since we can't judge other societies in different time periods any more than in different geographical regions, so thanks for relegating us to the dark ages there Quinny.)
PS: The comparison was not asinine because it's not a comparison. It's a thought experiment. A implies B which implies C, so what do we think about C? That's how philosophy works.