Does what I did—a movement similar to prostration before a naked woman, whether intentionally or not—constitute apostasy (kufr) necessitating sincere repentance, and how can I rid myself of the intrusive thoughts that prevent me from repenting sincerely?

It is not permissible for a Muslim to watch films that contain forbidden content, such as nude women, because it is a form of unlawful zina (fornication/adultery), in accordance with the words of the Prophet, peace and blessings be upon him: "The two eyes commit zina, and their zina is looking." Allah Almighty has commanded lowering the gaze, saying: {Tell the believing men to lower their gaze and guard their private parts. That is purer for them. Indeed, Allah is Acquainted with what they do} [An-Nur: 30].

Whoever watches these films must repent to Allah Almighty with a sincere repentance that fulfills its conditions: regret for what has passed, a firm resolve not to return to it, and sincerity to Allah Almighty in repentance. As for pathological obsessive thoughts, the person afflicted by them is not held accountable for what they do under their influence, but they are required to repent and stay away from the whispers of Satan. Sincere repentance erases all sins, and the repentant person should have a good opinion of Allah and the vastness of His mercy.