Parshat Metzora ends with the laws of impurity of bodily emissions (15:1-32), which are presented in the following order:
2. Seminal emission
It can readily be seen that these four laws form a chiastic structure:
1. Unnatural male emission
2. Natural male emission
3. Natural female emission
4. Unnatural female emission
The underlying logic is clear. The laws are divided into those of males and females, while the male and female sections are each divided into natural and unnatural emissions. The order of laws in the female section is reversed, so that as we go from the male to the female section, the subtopic (natural emissions) remains the same.
Like all ancient texts, originally the Torah was most often recited and heard rather than read by individuals. One can skip from page to page while reading a book, but not while hearing an oral recitation. Thus smooth continuity, in addition to logical structure, is a compelling factor in the Torah's organization. A chiastic structure was inevitable in this chapter, because no other ordering is continuous between its sections as well as logically clear.
It is on the basis of unambiguous passages like this that people think that the Torah uses chiastic structures. The key question is to what extent such structures are present in cases when the structure is less apparent and its utility less clear. That is one debate between the "literary Tanach" enthusiasts, and between those who think literary methods are sometimes overused to discover patterns that do not really exist.