Rinnie, with all due respect I believe you’re reflecting the views of some Protestants here, but not Catholic theology.
In Catholic theology, don’t you need the consent of the will to commit a
sin?
If there’s imperfect consent there’s a venial sin (if, for instance, one takes a momentary pleasure in the lustful thought before rejecting it); if there’s full consent there’s a mortal sin.
Temptation, in Catholic theology, does not automatically mean sin.
The passage from the Sermon on the Mount to which you refer is indeed understood by some
Protestants to mean that the mere occurrence of a lustful thought in the mind is sinful. But the passage says “look at a woman to lust after her,” which in the Catholic interpretation as I understand it implies a deliberate choice to lust.
Edwin