In this chapter, the LORD tells Moses the punishment for most of the crimes described in the past two chapters.
I don't feel it's necessary to talk about most of the punishments in this chapter because they relate to things I have already discussed and are otherwise simple. The punishment for most of the crimes listed here is death, which is consistent with what we have seen before. As before, the Israelites cannot imprison anyone because they still have to journey through the desert, making it difficult to continue feeding anyone who cannot contribute to the overall well being of the community as a whole. Combined with that is the general mandate to expunge sin from the community, because as we have seen with the ceremonial laws, impurity can be transmitted from person to person just like a disease.
I found it interesting that some of the punishments are specifically carried out by the LORD, rather than the people. For instance, v. 20-21 sentences barrenness for people who commit certain sexual crimes. While it's true that people can be sterilized in certain ways (e.g. castration), I think these verses are suggesting that the LORD would make the offending couple remain childless through their lives, rather than through some human mechanism. Also, earlier in the chapter v. 4-5 state that the LORD will carry out any punishment that men do not. This creates an interesting joint responsibility for the punishment of crimes between men and God, with the general implication being that God will punish any crime that men do not.
Verse 18 makes it clear that having sex during menstruation is punishable by death. The provision regarding ceremonial uncleanliness for the same act confuses me.
The end of the chapter, v. 22-26, restates the purpose of all these rules, which is chiefly to "to make a distinction" between clean and unclean, between the Israelites and other nations, and therefore between the LORD and all other gods. As I have said before, these points are related: it is by following a distinct set of customs, different from all the other nations, that the Israelites are to be distinguished, and it is by distinguishing the Israelites that the LORD is to be distinguished. Verse 23 makes clear that these are not arbitrary rules however, because the nations of Canaan and Egypt were doing things that the LORD "abhorred", so there is also a moral judgment.
In general terms, I think of the biblical laws as having three main purposes: 1) To establish a moral framework in keeping with the LORD's purposes, 2) to establish practical guidelines for the Israelites to follow, demonstrating wisdom and insight, 3) in keeping with the first two principles, to make clear that Israel is a nation set apart and above all the other nations. All of these are "means to an end", and the "end" is to restore the earth to a sinless state.
The customs and behavior of the Canaanite nations are rejected on the grounds of running contrary to the LORD's moral law.