Chapter 15 of the book of Vayikra records the tumah of the zav, shikhvat zera, the niddah and the zavah. These tumot all refer to emissions from the body, with two in reference to men (zav and shikhvat zera) and two in reference to women (zavah and niddah). Also, for both men and women, there is a normal emission, shikhvat zera and niddah, and an abnormal emission, zav and zavah. Yet, there are significant differences between the male and female types of tumah.
The zav and the zavah even share the same name, but it appears that the tumah of the zav is more severe than the tumah of the zavah. With regard to the zav, anything he sits on becomes tamei, and a person who touches such an item also becomes tamei for one day, 15:4-6. This law also applies to the zavah, 15:26,27. Also it appears that just like touching a zav makes one tamei so too touching a zavah makes one tamei, 15:7,19. However, the zav has additional secondary effects from his tumah that do not apply to a zavah as for example only the spit of a zav causes tumah, 15:8. Also by a zav, anything below where he sits, such as a saddle, becomes tamei, not just the items that come in direct contact with him, 15:9,10. Also, if a zav touches something with his hands but he did not wash them, then the touched item also becomes tamei, 15:11,12. (15:11 has generated much discussion, but I do not see why it cannot be understood as referring to a case where the zav touches an inanimate object with his hands, as before the Torah only refers to where the zav sits and if he was touched by a person.)
Accordingly, the zav has many more secondary effects from his tumah than the zavah has from her tumah. In addition, it appears that the purification of the zav also differs from the zavah, as the zav requires mayim chayyim, running water, 15:13, while this requirement is not mentioned by the zavah. The fact that the zav has more secondary effects from his tumah and undergoes a more burdensome purification process suggests that the tumah of the zav is considered more severe than the tumah of the zavah.
The tumot of the niddah and the shikhvat zera, the cases of the normal emissions, have the opposite pattern than that of the zav and zavah, since the tumah of the female, the niddah, is more severe than the tumah of the male, shikhvat zera. The tumah of the niddah has the same secondary effects as the zavah, and a woman who is a niddah is tamei for seven days, 15:19,25. However by the man, the only secondary effect of the tumah of the shikhvat zera is when the semen touches an item, and the man is only tamei for one day, 15:16,17.
Consequently, by the tumah due to normal emissions, the tumah of a man is less severe than the tumah of a female, while by the tumah of the abnormal emissions, the tumah of the man is more severe than the tumah of the woman. While it is possible that there are some underlying rationales that differentiate the reasons for the different tumot, the Torah does not posit any such differences. Possibly, the idea is that the net tumah of the male and females are roughly equal, as the combined tumah of the shikhvat zera and the zav would equal the combined tumah of the niddah and the zavah.
In addition, maybe the frequency of the occurrence of each tumah is also relevant to the levels of tumah. If one assumes that the zavah condition was more frequent than the zav, and that the tumah of shikhvat zera was more frequent than the tumah of a niddah, then maybe the reason for the different levels of severity of tumah is due to the differing frequencies of the tumah. (See Milgrom, 1991, p. 953, who writes “in ancient times, and indeed until the present age, women did not menstruate frequently during their childbearing years.”)
The tumah of the shikhvat zera is the most frequent tumah and hence it has the lowest level of tumah. Afterwards, the next most frequent tumah is the niddah, and her level of tumah is less than the zavah, as the niddah does not have to bring a sacrifice when she becomes tahor. The zavah has the third level of tumah, more than the niddah but less than the zav, and her tumah is more frequent than the niddah but less frequent than the zav. The zav then has the most severe level of tumah, but the most infrequent occurrence. If this is idea is correct, then the expected value of tumah (frequency * level of tumah) could be approximately equal in all four cases or maybe the total expected value of male and female tumot might be roughly equal.