What is the ruling on an unmarried young man imagining that he is having intercourse with his wife due to the strength of his desire?

Congratulations on your repentance, and we ask Allah to grant you steadfastness. What happens during sleep is not blameworthy, because the pen is lifted from the sleeper. As for imagining these matters while you are awake, it is impermissible; because it may lead to the forbidden. The Permanent Committee for Issuing Fatwas has answered that thinking about intercourse with the intention of ejaculation is impermissible. However, if the thought is involuntary, there is no blame in it, but one must perform ghusl if semen is discharged. A Muslim should refrain from intentionally thinking about these matters.