Do marital relations fantasies that lead to a certain sensation necessitate Ghusl or Wudu', and do they affect the validity of fasting in this case, especially if no penetration occurred?

We advise you not to dwell on these whisperings (waswas) and to know that fasting is not invalidated by discharge resulting from thought, whether it is madhy (pre-seminal fluid) or mani (seminal fluid), based on the Prophet, peace and blessings be upon him, who said: "Indeed, Allah has forgiven my nation for whatever crosses their minds, as long as they do not act upon it or speak of it." The default ruling for the fluid discharged after thought is that it is madhy, which necessitates washing it off and performing ablution (wudu'). A full ritual bath (ghusl) is only obligatory if the discharge of mani is certain.