Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.

Fantasizing about another person while having spousal relations with your marriage partners is mental Zina or adultery. As the Prophet, peace be upon him, said, “Eyes fornicate, the mind or heart fornicate, fornication of eyes is staring, and fornication of mind is lustful thoughts. The private parts finally consummate the act!” (Imam Ahmad)

As a bachelor, you should keep your mind away as much as possible from lustful thoughts, for they are doors to Zina. According to the principles of jurisprudence, ‘that which leads to haram is also considered as haram.”

Therefore, I would urge you to supplicate as follow:

Allaahumma tahhir qalbee wa ahsin farjee

(O Allah, help me keep my mind pure and help me keep my private parts chaste.”

Almighty Allah knows best.