Your insight "once a priest, always a priest" is a correct one. Thus, a man who, "leaves the priesthood," is not leaving the priesthood, per se, but is setting aside the practice and discipline of the priestly ministry. 
 
For a priest to validly and licitly marry in the Church, he must first be “laicized.” That is, while his priestly character remains, he is permitted and then required to live as a layman in the Church. And so, except in very rare “danger of death” situations, he cannot hear confessions, or give anointing of the sick, and in nowise celebrate the Sacred Liturgy or exercise other offices related to the priestly ministry. 
 
Further, when laicized, he is usually dismissed from the discipline of celibacy and free to marry. Note that celibacy is a discipline. And while a common and expected discipline of most priests, it is not utterly intrinsic to the priesthood. There are married priests, most of them in the Eastern Rites of the Catholic Church. But even in the Western (Roman) Rite there are some married priests. Most of these have come over from Anglican or Lutheran ministry and were then ordained Catholic priests. 
 
Thus, when a man leaves the active priesthood, and is laicized, he can also have the discipline of celibacy relaxed for him by the Church, such that he can marry. 
 
It is certainly lamentable that some men do leave the priesthood, a ministry they agreed to accept for life. And yet the Church, as the loving mother, does make some pastoral provision for men who regrettably have need of leaving the active priestly ministry for grave reasons. These provisions are in place so as not to utterly lose them to the practice of the faith, and to hold them as close to Christ and the Church as possible.

Your answer is not far from the point. People often give the Church exaggerated power, as though she can do anything she pleases. But, as the last three Popes have all stated, the Church has no authority whatsoever to confer priestly ordination on women. This is because Jesus, though he broke many conventions of his day, nevertheless called only men to be apostles. And in the call of the apostles is the origin of the priesthood. 
 
The Church can no more alter the matter and form of this sacrament than she can use beer and pretzels, for consecration instead of bread and wine. There are just some limits we must observe.