Lev. 15, bodily discharges

This chapter covers both normal and abnormal discharges from any genital area of either sex. Simply going to the bathroom is not dealt with as something unclean, but everything else is, including normal sexual activity between a husband and wife.

But didn't God ordain sex?

Yes, He did. There are a couple more points to consider, however:
1) In heaven there is no marriage, and thus no sex. This is a reminder of the position of man and woman in God rather than in each other. Thus what is relegated to only the earth is by nature unclean to some extent.
2) As a health measure. Certain diseases are easily transmitted through body moisture, especially in the sexual fluids, which are high in bacteria nutrients.

It should also be noted, however, that in this chapter normal sex and a woman's normal mentstrual flow were not reasons for sacrificial offerings. Those were for the offerings after one was healed of abnormal discharges of some kind.

The health reasons for isolation and ceremonial uncleanness during these times is obvious, I think. The spiritual reason is given by the Lord in verse 31:

You must keep the Israelites separate from things that make them unclean, so they will not die in their uncleanness for defiling my dwelling place, which is among them.

There is something to think about for us in connection with God's words here: As He dwelt in their presence, so the Holy Spirit dwells within us as Christians. He was concerned about the people remaining ceremonially clean -- which often had to do with what they touched or when they were sick. Do we take care to keep our own bodies away from things which dishonor the Lord as well? It's not up to me to judge, but I have to admit tobacco use and drug use comes to mind here...for many alcohol, and certainly the over-use of alcohol might also come to mind.

Or, if the meaning today is to be only spiritual, which is a possibility, then what about the thoughts we think and the things we pay attention to, 'touching' the unclean with our minds?

Just a thought...