When speaking about various theological and philosophical topics, such as anthropology and ethics, a major source of argument is the use of the term "unnatural." Many a modern man will argue that something is not unnatural because it is a phenomenon that occurs in the natural world.
This is not how the term in used in traditional theological and philosophical discourse. Rather, when we speak of nature, we are speaking of a thing's formal and final cause, insofar as the form of a thing is the principle that moves the thing towards its proper end. Thus, when we say that something is "unnatural," we mean that it causes a thing to act against attaining its proper end.
Thus, it can be entirely correct to call certain urges, inclinations and actions unnatural, even though they appear as phenomenon in the natural world.