It is not only false and irrational but actually dangerous to assert that because something is "natural" it is therefore also moral.
Simply because something happens in nature, or simply because animals are observed to perform some behavior in their natural setting, that it is somehow therefore morally acceptable for a human to perform the same behavior, is patently absurd and morally depraved thinking.
In short, it is false in at least two ways:
The first is that animals commit cannibalism, slavery, murder, war, even genocide. As such, if someone argues that "natural behavior" is okay because it occurs in nature, then one must also include by implication that cannibalism, slavery, murder, war, and even genocide are also okay, because they occur in nature and are equally "only natural."
The second way one gets oneself in trouble is because humans are also part of nature, and because they are equally a part of nature one implies that everything a human does can therefore be considered natural, and therefore moral. With this reasoning one again justifies rape, cannibalism, abuse, slavery, pedophilia, murder, torture, war, and genocide.
The first example is irrational because one has no criteria by which to deny any evil, and so there is likewise no criteria for determining any good either. One cannot proclaim something good in a system which cannot discern what a good even is, only what is natural - thereby not only letting in all the cruelty of nature, but actually justifying cannibalism, slavery, murder, war, and genocide. The second example suffers the same fate as the first, but is particularly grievous because it ends by making all human behavior self-justifying in its "naturalness."
In sum, one cannot rationally argue that because it is "natural" it is therefore okay, because then it is okay to do anything that animals do, or okay to simply do whatever terrible deed because it is simply a part of human "nature." This is irrational and completely undermines the entire moral force of an argument "from nature," actually disallowing any sense of morality, and dangerously justifying great evil.
Nature is no basis for determining right and wrong, and without right and wrong one cannot argue that ANY behavior is morally defensible.
