Only men can be priests because men can't give birth.Go on.
For women, their roughly 28-day menstrual cycle corresponds to a multiplication by 4 of the 7-day week: 4 times 7 days is 28 days.
Women shed both body and blood as part of that cycle.
The shedding process lasts an average of four days. Each of these four days corresponds to the four 7-day weeks, meaning that each of the four days corresponds to the Sabbath of each of the four 7-day weeks.
Like the Sabbath, women (in traditional cultures, and in the Torah) are to "rest" during the 4 days of menstruation.
Menstruation does not happen if they are pregnant (or, in other words, if they are about to take part in the 'incarnation' of a new human life).
Unlike women, men cannot claim the power to sacrifice, as part of their biological nature, body and blood during a quadruplicated 7-week cycle. Unlike women, men cannot claim, as part of their biological nature, to "rest" on a monthly (or, symbolically) weekly cycle. Men cannot claim, as part of their biological nature, to be able to give birth to a newly incarnated human being.
However, by entering the priesthood, a man is able to symbolically, or mentally, present the sacrifice of the body and blood of Christ, on a weekly basis, during the Sabbath, the day of rest, and to re-present the incarnation of the human-God Jesus Christ.
For women, religion is incarnate, it's physical, it's biological. For men, religion is theological, it's intellectual, it's psychological. Only if the two meet as one, does religion become "spiritual", which literally means "of the breath", because it's the breath that connects body and mind, incarnation and in-psyche-ation.