I have been listening to a series of recorded lectures by Fr. Brian Mullady, O.P. In one of them he talks about different moral theories that have arisen in the past century. Two of them are called proportionalism and the fundamental option.* As I understood him (going from memory since I don’t have the lecture in writing), proportionalism is judging the morality of an act by weighing its likely consequences in the light of the moral law, in order to choose the lesser of two evils, but denies that any act is always and everywhere sinful. It doesn’t deny the moral law, it just denies that the strictures of the moral law are applicable in every situation.
The fundamental option means that no individual act merits you either heaven or hell. What matters is your “fundamental orientation” towards or against God, which again, is not changed by individual actions. Which ultimately means that there is no such thing as a single act constituting a mortal sin.
Mullady says that both these ideas were popular, maybe even prevalent, during the 60s and 70s, especially in seminaries. Since many priests were taught and therefore believed that acts were not always and everywhere wrong, but their morality depended on circumstances, a priest could reason that an act of pedophilia, for example, might be wrong for someone else, but not for him in this instance.
Also, if no single act constitutes a mortal sin, then this particular act he is contemplating won’t necessarily endanger his soul, so long as he doesn’t change his “fundamental orientation” towards God. And so if he “needs” it at this particular time, or believes that it’s justified for this or that reason, then he could conclude that in this specific concrete situation in which he finds himself, an act of pedophilia is OK.
Whereas he could not have found any loophole allowing his actions under traditional moral theology, which would have told him that any deliberate sexual act outside marriage is always and everywhere gravely sinful.
*I claim no expertise in theology, moral or otherwise. As always, I post in the hope (among others) of being corrected where I’m wrong by those who know better.