What do Catholic nuns do? They are human, so they have to act, to pursue happiness. How can happiness be attained in the course of a 100% boring and purposeless life in which nothing happens? On a typical day, does a nun hang around? Does she stare at the wall? Does she occupy herself by watching the grass grow? What else is there to do in a convent? "How was your day, sister?" "Uneventful." And every day is like this.
Perhaps nuns aim to imitate Mary. But this is completely hopeless. We may grant that Mary has Perpetual Virginity; I find that entirely plausible. But she was married! To two "men": God the Father and Joseph. Again, she may not have had sexual relations with Joseph, but for all we know, they still cuddled. Far more incredible, she produced fruit. And what fruit! God the Son made flesh! Mary had adventures. She felt enormous joys for which she paid with equally enormous sorrows. Nuns pale in comparison.
Now regarding Mary's perpetual virginity, the Catholic Encyclopedia writes: "As to Mary's virginity after her childbirth, it is not denied by St. Matthew's expressions 'before they came together' (1:18), 'her firstborn son' (1:25), nor by the fact that the New Testament books repeatedly refer to the 'brothers of Jesus.'" Well, perhaps.
Regarding Mark 6:3, the Catholic New American Bible comments: "in Semitic usage, the terms 'brother,' 'sister' are applied not only to children of the same parents, but to nephews, nieces, cousins, half-brothers, and half-sisters... While one cannot suppose that the meaning of a Greek word should be sought in the first place from Semitic usage...," etc., concluding matter-of-factly, "The question of meaning here would not have arisen but for the faith of the church in Mary's perpetual virginity."
I think it makes a lot of sense, despite the disputed meaning. God the Father was her "husband," and it is bizarre to believe that He would share her with anyone, before or after Jesus' birth. But God would not let Mary stay in an unbecoming condition as a single mother or shouldering the burden of raising Jesus alone or without Joseph's patronage. Her human family had to be complete, and Jesus probably needed a human step-father, anyway, if only for physical protection as a child, such as against Herod (the angel of the Lord appeared indeed to Joseph with the message to flee to Egypt).
Finally, there is the "virginity" of the nuns. The Catholic Encyclopedia relates that "the Church... has always considered the state of virginity or celibacy preferable in itself to the state of marriage, and the Council of Trent pronounces an anathema against the opposite doctrine." This extremely dubious teaching is justified as follows: "The state of virginity means a signal victory over the lower appetites, and an emancipation from worldly and earthly cares, which gives a man liberty to devote himself to the service of God. ... experience bears witness to the marvelous spiritual fruit produced by the example of those men and women who emulate the purity of the angels."
What purity of the angels? The angels are no more pure than rocks. They, too, may be male and female, but they have no sex drive. They do not reproduce. They were not told, as both Adam and Jacob were, to "be fruitful and multiply." They literally cannot marry. And, as I have pointed out, human beings are explicitly distinguished from angels by having and requiring their bodies for a meaningful happiness, both here and in the life to come, and an active life. Contemplation in heaven and action in paradise for humans are united in a most sublime and spectacular way; but however pleasurable the contemplation of God will be in itself, its ultimate purpose will be practical.
Each human person is born woefully incomplete and requires, for any kind of tolerable existence, union with his or her complements. No one is born self-sufficient. Special God's grace may grant a reprieve from the sorrow for being unmarried, but this signals almost always a priestly vocation, a unique job, and has nothing to do with "victory over the lower appetites," which have always been meant to be controlled, not destroyed.
Here's what I mean: some people end up cultivating a peculiar form of charity, namely one that's more universal in scope. To combine it with the deeply particular love for and loyalty to a spouse and children may be difficult. One may decide to renounce marriage for the sake of his service to "humanity." However, neither kind of charity is superior to the other. A government bureaucrat is not morally better than an entrepreneur, even if the bureaucrat looks after some "public goods." A modern "social justice warrior" is far more likely to be a contemptible creature than a man who minds his own business.
It may be argued that a Catholic clergyman ought to exhibit precisely this sort of "universal" charity. But again, a male priest does actual work. He celebrates mass, he hears confessions for the sake of the people. What, to reiterate, do nuns do?
In short, service of God consists in serving fellow man and improving in virtues as a result. But man finds happiness in the active life, including marriage. "Service" to others is then ordered toward and for the sake of this sort of happiness. The means are inferior to the end; hence the service of the religious, just as the service of everyone else, consists in promoting the true happiness of mankind in all its forms. Simply to renounce the world and the "worldly and earthly cares" is a monstrous act of contempt for one's mission in life. If you hate living here, what makes you think you'll suddenly develop a taste for life in the hereafter? What natural drive is stronger than sexual desire? If you casually despise that, then you are indifferent to life, and then who are you? A robot that just sort of goes through the motions while waiting for death? What is there about you that's worth saving? I wonder how many nuns ask themselves this very question.