Why isn’t a sacrament when women become nuns, but it is when men are ordained priests?
A sacrament is an outward sign instituted by Christ. Jesus determines what a sacrament is and who will receive it. As for the priesthood, it is not comparable to religious life. In fact it’s not comparable to any other thing on the earth. It is a share in Christ’s priesthood which he himself established. No one is worthy of it. A Catholic priest actually ministers in the person of Christ. That Jesus ordained only men would be unfair if all human beings had a right to such an honor. The fact is, we don’t.
Religious life, on the other hand, is a way of living that is based on the evangelical counsels that Jesus preached. The vows of religious life represent the totality of human existence. We are all called to be chaste (both married and single), we are all called to obey lawful authority, and we are all called to be poor in spirit. Religious life accentuates these by going a step farther, thereby drawing attention to them. It acts as a beacon, reminding people that we all owe God the oblation of all that we are. Religious life is opened to men and women alike.