When a priest I thought very spiritual left the priesthood to marry, I was very disappointed and spoke with another priest friend. He told me we must pray for him and all priests who leave, of course, and also added words to the effect that 'the world is too strong for them' -- they can't resist it. And that reminds me of a scene in a movie that impressed me with this powerful sway of the world's ways.
In Lawrence of Arabia, T.E. Lawrence and his troops have just come upon a village that was thoroughly destroyed, everyone slaughtered. When they see the tribe that has done this escaping in the distance, in the desert, his men want their revenge, especially since one of them came from that village. Lawrence, who hated killing and had passionate mercy in the beginning of his adventures, doesn't want to go after the perpetrators with such vengeance--they have a mission elsewhere--but one by one, in groups and with shouts of revenge, the men have begun riding off on camels to catch up and kill the retreating enemy. Lawrence tries to stop them at first, but the rhythm and fury of their shouts, the anger he feels over the bloodshed he has just seen, slowly get to him, and one sees the pull of the world's ways taking hold of him, until he is captured by it, and shouts "No prisoners!" -- riding off with his men. They complete the world's ways by doing the same to the enemy that was done to the village. The pull of the world is strong in all emotions.
I've always loved a good story, a good example cleanly shown. This, in essence, is what happens to many priests. Whether it is leaving to marry, or worst of all, abusing children and taking advantage of a parishoner's trust, they have ridden off. . . and indeed, we must pray for them.