But what if fatty foods were already banned for some other reason? Wouldn't the correlation to colon cancer be yet another reason to keep fatty foods banned?
Isn't that what we're talking about with marijuana?
"But I would never make a factual claim that adultery destroys marriages without collecting some evidence, defining my terms, and examining some alternative explanations. And, no, I would never just utter a certainty, as you have done, on the basis of "common sense."
But if I approached you and asked you if we should vote nationwide to allow adultery and remove the social stigma associated with it, wouldn't "common sense" be enough for you to say no? Would you need a study before you could respond to that question?
Moreover, a referendum such as the one you propose cannot removed a "social stigma." In this country, it is legal to be a practicing homosexual. It is "allowed." Has the social stigma disappeared? It depends on where you live. Likewise, it is legal to be married to a person of another race - but there may still be social stigmas associated with it. Stigmas cannot be removed by fiat - only by practice and acceptance. That is why different regions will have different cultures and societies.
As far as to whether my "common sense" - by which you, I think, mean my own personal inclination - would want me to preclude even a vote on such an issue - why would I do that? Because I would be afraid that the consensus would differ from my own opinion? If I can't support my position with facts and reasoning that will win over my opponent, then perhaps my position isn't a legal issue at all. Moral or ethical - but not an issue of law. I can't advocate forbidding debate, or even a referendum, on a topic because I fear the majority may not agree with me.
As for the reverse - outlawing adultery - I would never do so simply on the basis of "common sense." I would need some evidence before I enforce my own personal belief on all of my fellow citizens as a secular law. That is what a civil society is about - that we reach a consensus and base our laws on that consensus and the Constitution.
As far as the "well it's already banned" argument against marijuana .... what was the original reason that it was banned in 1934? If the debate regarding its outright abolition ever comes up again, perhaps we can review the original reasons that it was criminalized, as well as any additional information about it.