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).
I think that is a fair point but it doesn't negate the conclusion of how the Vatican relates to sin. Sin must be acted out. Thus a man isn't really sinning if he has impure thoughts towards someone other than his wife. He is only sinning if he acts upon those thoughts. All you have to do is go back through my exchanges and you'll see this is the interpretation of the Church.