What I mean by Natural Law is a system of ethics that was promoted by Plato and later Christianity. This system holds a belief in realism, that the world around us is real and has an essential nature. Whether it be the Forms or God, there is an absolute measure of truth and justice. Natural Law within Christianity can be traced back to Paul, but it was Thomas Aquinas' fusion of Aristotelian philosophy and Christianity that strongly fueled Natural Law ethics for many more centuries.
Within this view, God is the source of truth and purpose. Therefore, every action a human takes must align with that purpose. Sex, for example, was only part of the divine purpose when used for reproduction. The carnal self must be denied; the temporal self, the soul, is what fulfills God's purpose. If you read Aquinas, he details many sexual acts, including homosexuality, which are not natural because they don't fulfill God's ethereal purpose.
That position has been widely debated over the centuries, and many religions have relaxed their views on what is allowed, but they do this by reinterpreting the Bible. They still hold that right and wrong is determined by God, not by humans.
I see this less and less in our legal system. Laws that attempt to regulate morality are based less on absolutes and more on what is beneficial or preferable to the majority. This is why people of different races can marry. Women may take birth control. Divorce is common. Gay marriage is gaining ground. And inversely, children are protected from pedophilia (there was nothing in Natural Law protecting a 12-year-old girl from marriage and pregnancy), slavery is outlawed, and restrictions on cigarette smoking are increasing. The fact that laws are changing, that they are even capable of changing, and are not based on religious tradition is evidence of a Utilitarian influence.
There are still some remnants of Natural Law in our legal system, but I see them fading over time.
You said: How is it that cigerettes, with their mountains of provable societal detriment, are perfectly legal to buy, virtually anywhere, whereas I will be placed behind bars, if caught, if I walk in the forest to collect hallucinogenic mushrooms?
I'm certainly not stating that every law on the books is fair or just or right, but simply that the framework allows for a relative approach to ethics. Twenty years ago, cigarette companies advertised on TV and the harms of smoking were swept under the rug, while you could be imprisoned for possessing marijuana. Laws on these have shifted greatly over the past couple decades. There are many bans on cigarettes and a ton of negative media attention. There is more public awareness of smoking cessation than there is advertising for cigarettes. Marijuana, on the other hand, is gaining ground. It's now legal in certain places and for certain reasons, and there is a big push to get it legalized. Is it possible that another ten or twenty years down the road, pot will be legal and cigarettes won't? This is a perfect example of how Natural Law is fading into the background, being replaced by a more relative ethical system. I would also agree that money plays a factor in just about every aspect of our society, but that's capitalism for yah.