One of the reasons why many theologians have been attracted to the theory of a "fundamental option" is that it seems in certain respects to correspond better to real-life experience. If we consider visible human relationships, between two married persons for example,we don't find persons who go frequently back and forth from being totally committed to each other, to selfishly rejecting each other, back to total commitment, and so on. As a rule, the relationship will overall either be gradually improving or deteriorating, and individual quarrels don't completely disrupt the relationship. If a man ceases to love his wife on account of a single dispute or fight, that would be taken as a sign he didn't really love her in the first place, or that he had been neglecting his love for her, letting various selfish interests break it down, and that the quarrel is only the terminus of a long process.
Leaving aside the theological aspect, based on God's covenant with man, the supernatural virtue of charity, etc., what is happening when a person resolves to live a certain manner of life, and yet occasionally, or perhaps frequently, performs concrete acts that are inconsistent with that? For example, he resolves to live for his family, and looks for his happiness within his family (and in the wider sense, within his society). But he sometimes performs acts, such as staying out excessively late drinking with his fellow workers, that don't make sense within that framework, but only make sense on the supposition of a preference for something else above happiness as a member of his family, within a framework in which what he wants is the ultimate point of reference, where self-love is the final measure. Or in other words, in terms of the end/means relationship, certain acts he does objectively can't be truly ordered towards the end of family happiness as a final goal, but only towards some other end, that may be thought of more vaguely (living a pleasant life), but that at any rate is some other final goal.
One might explain the situation in several ways:
1. The ultimate goal of his life is, and remains, the common good of his family and his happiness within that family, though he performs individual acts that don't make sense and are unreasonable in terms of that goal.
2. The ultimate goal of his life is all along some vague goal (living a good life, a social and pleasant life, or the like), in which his family's good is one element among others, though an important one.
3. The ultimate goal of his life is the good of his family most of the time, but when he does acts objectively contrary to that goal, then when he is doing those acts, he thereby re-orientates himself towards another final end, and remains directed towards that end until he makes a fresh resolve to live his life as a whole for his family.
The first and second explanations do seem, on the face of it, more plausible interpretations of what is going on than the third.
If we accept the first explanation, what would that mean in terms of the voluntariness of the man's acts? Since whatever one wills, one wills for the sake of one's final end, the man is therefore willing, for the sake of his final end, to do something actually inconsistent with that final end. This would seem to imply that his act is not perfectly voluntary, at least not in the respect in which it is contrary to his ultimate end. (This distinction, though it is often forgotten or overlooked, is an important one. An act may be completely voluntary in the sense that it is the result of a very conscious and explicit choice, without all of the goodness or badness of that act necessarily being voluntary).
The analogy one might make with love for God and the performance of sinful acts is, I think, clear. For the time we will continue to leave aside the issues pertaining to charity as an virtue infused by God and dependent on grace. A person who resolves to life his live above all for God, and yet on not so infrequent occasions does things objectively inconsistent with taking God as the rule for one's life, is like the man in the example above, who resolves to live for his family, and yet fails at certain times to act in consistency with that resolve. If one held that he continues, in fact, to be seeking God as his ultimate end, it seems one would similarly have to say that the acts objectively inconsistent with that end, or at any rate the badness and stupidity of those acts, are not entirely voluntary. Thus they would constitute venial sins by reason of a lack of full voluntariness.
But on this account, would any acts be fully voluntary? I'll return to this question in another post.