From masturbation to racism, from organic food to free range cows, from shampoo to all-purpose cleaners, from beards to smoking pot, from home births to refusing vaccines, the word natural has been employed to convince people that something is okay because....it's totally natural dude. Here is the magical formula:
x [behavior you like] + "is natural" = x is morally permissible/good
Masturbation. We say it's natural because the human body practically requires it, especially during young adulthood. The evolutionary and biological forces are so strong that it would take an enormous amount of moral will power to not do it. Kant, a product of his times in this respect, actually produced a moral argument against it, saying that masturbation amounts to muddying up your imagination, using your mind as a means to an end. Suffice to say we don't buy that argument, for good reason. In this case, the natural impulses far outweigh any moral arguments you can come up with, and the moral arguments are pretty weak. Leave it to the monks I say.
Sex. It's perfectly natural for human beings to have sex with many people, and even for partners to be unfaithful. It's so common that a zoologist, for example, would have to conclude that we are not in face a monogamous species after all but somewhere in the middle (explains our divorce rate). Yet, morally speaking, we do not condone sleeping around or cheating; not even close. Why? I think it's because sex also happens to be a moral issue, not just a biological impulse. Both are real. In fact, the moral argument is strong, and the natural inclination is not so strong that we cannot defeat it. In other words, it's in the moral sweet spot, in between natural inclination and moral inclination. Personally, this is how I harmonize my moral and biological natures: Sex is a special thing to be done with people you care about, and you should never cheat on your partner, ever. Just my opinion.
In fact, this really gets to the essence of morality. So many moral issues fall within the moral sweet spot: drugs, drinking, stealing, lying. They all involve natural impulse battling moral principle. Shout out to Kant for nailing it, among others.
knowing where it came from.
Is "natural" food better for you? First of all, pretty much all the food we eat has been modified by human beings, so that has no meaning. Second, just because a food is processed or genetically engineered doesn't mean its bad. Some people talk about "chemicals" as if they are bad things. If you like to grow corn and eat it, that's great, but the food itself is no better than corn at the grocery store. I think the truth lies here: many frozen foods, packed with preservatives, have way too much sodium and other stuff. Also, if you don't know what your food is made out of or where it comes from, there's a chance its bad for you, or perhaps we could say even a tendency due to corporate greed with values profit over nutrition. Eat local. Also, fast food is obviously bad for you. We all know that by now.
Same thing with pot or mushrooms. The fact that you can grow it, or that it comes from the ground, has nothing to do with how bad or good it is for your brain and body. I'm guessing the jury is out: they are in fact bad for you. Stop rationalizing getting high and just get high. I make my own beer, but I certainly don't rationalize getting drunk (I just do it).
the way it is
Racism. The main argument used for slavery was that the natural order of things required it; black people naturally are subservient animals and therefore should be treated as such, for their own good. Setting aside the factual errors, this is a very dangerous form of moral argument. You describe something in nature and then prescribe it as moral, as right, as the will of the Creator. This is a huge error, a category mistake. Scientific-types are especially prone. Just because something is does not mean that it ought to be. Simple as that. Morality challenges the status quo, demands improvement always. So even if black people were intellectually lower than white people at that time (due to slavery of course), then owning them does not follow. The moral question does not rely on such facts. Morality relies on principles, which hopefully are self-evident.
Sometimes natural means "normal." When kids are bad, for example, or act out occasionally, we console parents: "Don't worry, it's perfectly natural at that age…". My wife told me that lawns are not natural. Well, certainly they are normal. But what she meant was that lawns are dead zones, they don't promote growth; that not having a lawn is better for nature (flowers, water, butterflies, life). Makes sense to me. Marketing campaigns use the word "natural" simply to sell stuff. Factually it means nothing, it's a fad. Philosophers in the past used the word "natural religion" to describe religion without the Bible, sans revelation--supposedly the religion a person would come to "naturally." In sports we call someone a "natural" when they possess talent at a very young age. Then we've got the crazy anti-vaccine people and the home-birth people, who usually are rich, white, privileged folks who apparently think that modern medicine is unnatural or wrong or whatever. I'll end there.