Marriage and religious life are two fundamental ways to fulfill the fundamental vocation of every human being to love. Why is marriage a sacrament and consecrated virginity or celibacy is not?
Since Christ certainly could have made consecrated virginity a sacrament, any answer can only be based on arguments of appropriateness. Both marriage and virginity are signs of the union between Christ and the Church. Is there a difference in the way in which they are signs of this union, such that marriage is fittingly a sacrament, and consecrated virginity is not?
Marriage signifies the union of Christ and the Church inasmuch as the very union of the two humans spouses derives from, participates in, and is a likeness of the perfect union of Christ with the Church. Nevertheless this union of the spouses remains distinct from this spousal union of Christ and the Church. The spouses do not give themselves directly to Christ, but to each other.
Consecrated virginity signifies the union of Christ and the Church inasmuch as the virgin is devoted, by her own will and by the Church, to the very union that constitutes the Church, to the fulfillment of the union with Christ begun in baptism. Thus it bears less the character of a sign, and more that of the reality itself.
We might tentatively say, then, that it would be less fitting for consecrated virginity or religious life to be a sacrament, a sacred sign that confers grace, because it is above all reality, a deepening of the baptismal grace, the spousal union with Christ the Bridegroom. It is a sign of the future kingdom, but it is a sign of it inasmuch as it already anticipates it in this life.
Another, complementary way to explain this looks at the different ways marriage and religious life relate to time and history. Marriage pertains above all to the working out of God's plan for man in time. Those who rise from the dead "neither marry nor are given in marriage"; human marriage ceases with death, even though some aspects of marriage, e.g., the love between the spouses, endures beyond death. The virgin's spousal union with Christ, however, does not cease with her death, but is consummated—she becomes even more perfectly that which she began to be on earth through baptism and through her vows: the bride of Christ. Since all of the sacraments pertain to the dispensation of God's grace in time and history, it is thus more fitting for marriage to be a sacrament than for consecrated virginity to be one.