Is imagining intercourse considered adultery or marriage, and what are the resulting rulings, such as the prohibition of marrying the son of the one who imagined it, and is repentance an expiation for that?

Merely thinking about fictional stories is not a sin, but a waste of time. As for sexual fantasies, they are a gateway for Satan and a path to temptation, and are considered a form of figurative adultery, according to the Prophet, peace and blessings be upon him, who said: "The heart yearns and wishes, and the private parts either confirm or deny that."

Mulla Ali al-Qari clarified that if a desire settles within one's inner self and is not repelled, it is called zina (adultery) and becomes a sin that incurs punishment.

If you have repented from these fantasies, do not dwell on them, as they do not lead to what you mentioned, such as the spread of marital prohibitions or expiation, and so on. Rather, it is one of Satan's insidious approaches. Ibn al-Qayyim advises: "Guarding thoughts and preserving them, and being wary of neglecting them and indulging in them, for the root of all corruption comes from them."