Question:
Answer:
Given that immodest looks and related impure thoughts involve violations of the Sixth and Ninth Commandments, they always involve grave matter (CCC 1858). However, there are two other components of mortal sin: full knowledge and deliberate consent (CCC 1857; see 1854-64).
Jesus exhorts us to be vigilant re: sexual immorality, directing us to “nip in the bud” even impure thoughts: “Every one who looks at a woman lustfully has already committed adultery with her in his heart” (Matthew 5:28).
One wants to avoid scrupulosity in such matters but also moral laxity, as “fantasy” is sometimes incorrectly argued as harming no one. Such fantasizing harms first the person who engages in it, as related impure thoughts undermine their relationship with God while attaching them to—to one extent or another—a form of sexual vice. And such impurity harms the people with whom they interact in this regard, again to one extent or another.
If you have further questions on this matter, consult a good faithful priest in your area.