Is watching pornography for the purpose of learning matters related to the relationship between spouses considered a sin?

Allah the Almighty has forbidden looking at what is unlawful, such as pornographic films, as He says: "Tell the believing men to reduce [some] of their vision and guard their private parts." These films are not permissible under the pretext of learning about sexual relations, as this knowledge can be gained from books of Sunnah and Fiqh. What you did was a great mistake, but if you repent and regret it, Allah forgives all sins, as He says: "But indeed, I am the All-Forgiving to whoever repents, believes, and does righteousness, and then is guided," and His saying: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah.'" The conditions for repentance are: abandoning the sin, regretting it, and resolving not to return to it.