Am I bound by an expiatory offering for an oath after swearing that I have never watched pornographic films, even though I watched them previously before my repentance?

One must conceal one's sins when falling into disobedience, and it is permissible to lie to achieve this, such as denying the commission of the sin. If a person is compelled to swear falsely to conceal themselves or others, they are excused, and there is no sin upon them. However, if they are not compelled to swear, it is not permissible for them to swear falsely. If they do swear, they should seek Allah Almighty's forgiveness. Some jurists hold that expiation is obligatory for one who swears falsely.