I am curious, it seems that immoral sexual behaviors that are associated with Karet also receive the death penalty. Why is it that relations with a niddah receives Karet but no death penalty? This seems to be an exception.
You are correct in your observation. Although having relations with a nida carries the punishment of karet, it is not as severe as having a relationship with a family member or a married woman. Family members and a married woman are not people that a person can have a marital bond with, as we are not allowed to marry them. A nida on the other hand is a woman that one may marry, in fact the prohibition of having relations with a nida apply most to a married couple. The person is right, just the timing isn’t correct for having relations. Tis may be in part why the punishment is not as severe. Another point. A child born out of a forbidden relationship, i.e. the union of a brother and sister, is a mamzer, and may not marry a regular Jew. However, the child born from the union of a man with a nida is called a pogum, and a Jew is permitted to marry them.