Is the fast of one who experiences a wet dream during a Ramadan day valid, knowing that he did not intend the wet dream?

What you mentioned about yourself regarding abandoning prayer outside of Ramadan (definition) is a very serious matter. Therefore, you must repent sincerely and maintain your prayers. As for the fluid that comes out of you when you watch pornographic scenes, it appears to be madhy. We have already explained its ruling and how to purify oneself from it. If the fluid that came out of you during sleep was madhy, then the ruling is as previously stated. But if it was maniy (semen), then ghusl (definition) (ritual bath) is obligatory upon you, and a wet dream does not invalidate the fast. If you are uncertain whether the discharge is maniy or madhy, some scholars hold the view that one has the choice between the two rulings.