What is the ruling on sexual fantasies, and what is the solution to this problem?

It is not permissible for you to contemplate intercourse with any man other than your husband with whom you have a valid marriage contract or who has consummated the marriage with you. Since you are not married, this is not permissible for you under any circumstances.

You must repent to Allah the Almighty for the previous thoughts by abandoning them, regretting them, and resolving not to return to them. We also advise you to keep company with righteous women, to abandon places of temptation and doubt, and to seek help in preserving your chastity by lowering your gaze, frequently remembering Allah, continuously pondering the affairs of the Hereafter, occupying yourself with what is beneficial and fruitful, and abandoning everything that is harmful and causes loss.