Zavim, Chapter Five, Mishnah Eleven
1) He who has had a seminal emission is like one who has touched a dead sheretz.
2) And one who has had sex with a menstruant is like one who has suffered corpse uncleanness.
3) But one who has had sex with a menstruant is more stringent in that he conveys minor grades of uncleanness to what he lies or sits upon, so as to make foods and liquids unclean.
Section one: This mishnah compares the impurity levels of various people.
A man who has had a seminal emission has the same rules of impurity as one who touched a dead sheretz. As we learned in yesterday’s mishnah, such a person has first degree impurity.
Section two: But a person who has had sex with a niddah, a woman while menstruating, has a higher degree of impurityhe is a father of impurity.
Note that all of these laws are learned from midrashim on the Torah. They are in no way value statements, saying that having a seminal emission is akin to touching a dead sheretz or that having sex with a menstruant is worse (although it is forbidden). The mishnah is only interested in comparing levels of impurity.
Section three: A man who has sex with a menstruant has certain purity stringencies related to him that do not relate to a person who has corpse impurity. The former conveys a light form of impurity to anything he sits or lies upon in order for it to defile food or liquids (see Kelim 1:3). A person with corpse impurity does not defile in this manner, unless he touches that which he sits or lies upon.