Do you have a reference for that? The Vatican view on sin is that it must be "acted" upon. Here is a portion of a very lengthy article on the Vatican view of sin. I would especially note how "free will" is intertwined with this misguided theology.
Contrary to the teaching of Baius (prop. 46, Denzinger-Bannwart, 1046) and the Reformers, a sin must be a voluntary act. Those actions alone are properly called human or moral actions which proceed from the human will deliberately acting with knowledge of the end for which it acts. Man differs from all irrational creatures in this precisely that he is master of his actions by virtue of his reason and free will (I-II:1:1). Since sin is a human act wanting in due rectitude, it must have, in so far as it is a human act, the essential constituents of a human act. The intellect must perceive and judge of the morality of the act, and the will must freely elect. For a deliberate mortal sin there must be full advertence on the part of the intellect and full consent on the part of the will in a grave matter. An involuntary transgression of the law even in a grave matter is not a formal but a material sin. The gravity of the matter is judged from the teaching of Scripture, the definitions of councils and popes, and also from reason. Those sins are judged to be mortal which contain in themselves some grave disorder in regard to God, our neighbour, ourselves, or society. - New Advent on Sin
I would point out that the article notes how this differs from the Reformers who viewed sin as ANY action, internal or external. Evil thoughts that proceed from the heart, however hidden they are to those around, are considered sinful before God. Whether you are lusting after your neighbor, your neighbor's wife or his car is no different; sin is still sin and must be confessed before God. (I will caveat that there are degrees of sin with some sins being more vile in the sight of God.)
I should have know man's "free will" should be at the bottom of all this. Man can have any thoughts or ideas they wish as long as they don't act them out. Thus, according to this definition you could hate your neighbor in your heart as long as you don't "act it out". This is man's free will??? I'm always amazes me at how many theological errors are spawned over this ONE seemingly innocent belief on the free will of man. What a piece of garbage theology, IMO.
Then again, you'd think that I would learn my lesson.
But here goes: I think you're misunderstanding what we mean by "act". IMHO there are internal voluntary acts as well as acts that can be seen by others. Planning a deception is a sin, for example.
From the page to which you linked:
That sin may be committed not only by outward deeds but also by the inner activity of the mind apart from any external manifestation, is plain from the precept of the Decalogue: "Thou shalt not covet", and from Christ's rebuke of the scribes and pharisees whom he likens to "whited sepulchres... full of all filthiness" (Matthew 23:27).
Yes, we have free will and are responsible for our actions.
A doctrine of no free will is more problematic - it collapses IMHO.
If you don't have free will, you cannot, by definition, choose.
Anything.