The Appeal to Nature is a common fallacy in political arguments. One version consists of drawing an analogy between a particular conclusion, and some aspect of the natural world -- and then stating that the conclusion is inevitable, because the natural world is similar:
"The natural world is characterized by competition; animals struggle against each other for ownership of limited natural resources. Capitalism, the competitive struggle for ownership of capital, is simply an inevitable part of human nature. It's how the natural world works."
http://www.csun.edu/~dgw61315/fallacies.html#Nature, appeal to
This is the fallacy of assuming that whatever is "natural" or consistent with "nature" (somehow defined) is good, or that whatever conflicts with nature is bad. For example, "Sodomy is unnatural; anal sex is not the evolutionary function of a penis or an anus. Therefore sodomy is wrong." But aside from the difficulty of defining what "natural" even means, there is no particular reason to suppose that unnatural and wrong are the same thing. After all, wearing clothes, tilling the soil, and using fire might be considered unnatural since no other animals do so, but humans do these things all the time and to great benefit.
The appeal to nature appears occasionally in debate, often in the form of naive environmentalist arguments for preserving pristine wilderness or resources. The argument is very weak and should always be shot down. It can, however, be made stronger by showing why at least in specific cases, there may be a (possibly unspecifiable) benefit to preserving nature as it is. A typical ecological argument along these lines is that human beings are part of a complex biological system that is highly sensitive to shocks, and therefore it is dangerous for humans to engage in activities that might damage the system in ways we cannot predict. Note, however, that this approach no longer appeals to nature itself, but to the value of human survival.
http://www.infidels.org/library/modern/mathew/logic.html#natural
However, let us set aside such doubts about the category of 'the natural' for the moment and just ask, even if we can agree that some things are natural and some are not, what follows from this? The answer is: nothing. There is no factual reason to suppose that what is natural is good (or at least better) and what is unnatural is bad (or at least worse).
http://www.fallacyfiles.org/adnature.html
Nature merely shows us what is. It does not show us what should be. Cancer is a natural part of aging. Chemotherapy and radiation therapy is an unnatural but relatively successful treatment. It is not natural for an encyclopedia to be used as a booster seat at a dinner table. It was never “intended” to be a booster seat, but it helps.
By the way, no one is calling for replacing heterosexuality with homosexuality.