Is it a mortal sin for a priest to leave the priesthood?
If the answer is "yes", then my second question is: If is is a mortal sin for a priest to leave Holy Orders, then why does the Church abet in the priest's sin by granting laicization?
It's actually a form of apostasy, so it's a wee bit different than mortal sin.
Why would it be a form of apostasy? Apostasy means the formal abandonment or renunciation of one's religion. A priest who leaves the priesthood is not necessarily abandoning the Catholic faith. Some leave the priesthood because they have lost the faith, I admit. But some priests want to be released from their vows because, for instance, they have gotten involved with a woman and want the freedom to pursue a relationship with her and marry her. But that does not mean that because they committed the sin of gettig emotionally involved with a woman and now want to marry her, that they no longer believe in the truths of the Catholic faith.
Here's the easy answer:
http://www.newadvent.org/cathen/01624b.htmAPOSTASY AB ORDINE
This, according to the present discipline of the Church, is the abandonment of the clerical dress and state by clerics who have received major orders. Such, at least, is the definition given of it by most authorities. The ancient discipline of the Church, though it did not forbid the marriage of clerics, did not allow them to abandon the ecclesiastical state of their own will, even if they had only received minor orders. The Council of Chalcedon threatens with excommunication all deserting clerics without distinction (Hardouin II, 603). This discipline, often infringed indeed, endured throughout a great part of the Middle Ages. Pope Leo IX decreed, at the Council of Reims (1049): "Ne quis monachus vel clericus a suo gradu apostataret", all monks and clerks are forbidden to abandon their state (Hardouin VI, 1007). The Decretals of Gregory IX, published in 1234, preserve traces of the older discipline under the title De apostatis, which forbids all clerks, without distinction, to abandon their state [V, title 9, i, iii (Friedberg, II, 790-791) ]. Innocent III had however, at an earlier date, given permission to clerks in minor orders to quit the ecclesiastical state of their own will (Decretals of Gregory IX, III, title 3, vii; see also x, Friedberg, II, 458-460). The Council of Trent did not restore the ancient discipline of the Church, but deemed it sufficient to command the bishops to exercise great prudence in bestowing the tonsure, and only laid the obligations involved in the clerical state on clerks who have received major orders and on those who enjoy an ecclesiastical benefice (Session XXIII, De Reformatione, iv, vi). Whence it follows that all other clerks can quit their state, but, by the very fact of doing so, lose all the privileges of the clergy. Even the clerk in minor orders who enjoys an ecclesiastical benefice, should he wish to be laicized, loses his benefice by the very fact of his laicization, a loss which is to be regarded not as the penalty, but as the consequence, of his having abandoned the ecclesiastical state. These considerations suffice, it would seem, to refute the opinion maintained by some writers [Hinschius, System des Katholischen Kirchenrechts (Berlin, 1895), V, 905], who think that a clerk in minor orders can, even at the present day, be an apostate ab ordine. This opinion is rejected, among others, by Scherer, [Handbuch des Kirchenrechtes (Gratz, 1886), I, 313; Wernz, II, 338, note 24; Hollweck, 299].
Today, after three ineffectual notices, the apostate clerk loses, ipso facto, the privileges of clergy [Decretals of Gregory IX, V, title 9, i; title 39, xxiii, xxv (Friedberg, II, 790 and 897)]. By the very fact of apostasy he incurs infamy, which, however is only an infamy of fact, not one of law imposed by canonical legislation. Infamy involves irregularity, and is an offense punishable by the loss of ecclesiastical benefices. Finally, should the apostate persist in his apostasy, the bishop may excommunicate him [Constit. of Benedict XIII, Apostolicæ ecclesiæ regimine, 2 May, 1725, in Bullarum amplissima collectio (Rome, 1736), XI, ii, 400].