The single life is also a legitimate vocation: it should not be the norm, but trads who condemn it so hastily and arrogantly and Priests who are too incompetent to treat it in the proper light (either in the Confessional or at the pulpit) make it as if trads committed to the single life are somehow defective.
It's not a legitimate vocation is someone is dating at the same time. They should live a chaste life.
This is very true: for one who is committed to the single life, celibacy is a process of self-detachment and mortification, and if one entertains the notion of "what if" or "maybe I'll go and see what it's like to date" then one is not celibate: abstaining from sex acts by default is not the same thing as Christian virtue of chastity, especially when one exposes himself to occasions of sin by "dating" without intent to marry.
A momentary lapse may be absolved by the Sacrament of Sacred Penance and by works of penance and reparation, but these lapses must be
an occasion for one to enter into a more profound self-examination so that one may avoid future lapses and truly comprehend the sacrifices demanded by chastity. Otherwise, one would fall into the pernicious habit of lapsing, Confessing, praying the imposed penance, and days later repeat the process: this devalues celibacy and gives a perilous occasion to the cultivation of a tepid heart and lax conscience. Chastity involves the continual and ever-progressing mortification of the will and of the senses, an unceasing purification of the self so that it strives to a greater and more ardent charity, both fraternal and divine. The solitude of chastity is not the loneliness or desolation of those who are culpably inadequate for a healthy relationship that has marriage for its end, but rather a supernatural integration of self and the union of the cohesive self with God in total self-abandonment: this in turn naturally leads to fraternal charity as manifested in corporal and spiritual acts of mercy.
Celibacy is the "default setting" of any Catholic worthy of the name. Sacred matrimony
alone endows a man and his wife with the privileges of availing themselves of their nuptial rights, whilst the Priesthood and Holy Religion transfigure and elevate celibacy into consecrated virginity/chastity. The single life, though bereft of the blessings of marriage and the graces of consecrated chastity, can indeed be spiritually fruitful if one endeavors to practice the theological and moral virtues and persevere and progress in such endeavor so that one may be detached from self and things to serve God and those whom He loves in a greater way.
Fr. Dominic Unger, O. F. M. Cap., wrote about the single life in his book
The Mystery of Love for the Single (Chicago: Franciscan Herald Press, 1958; reprined by TAN), and based his teachings on the Encyclical Letter of Pope Pius XII
Sacra virginitas (25 March 1954). If one wishes to know about how the single life should be viewed, this is the book to read.