Due to the corrupt and immoral environment most people are living in these days, exposure to all kinds of shameless filth in the form of videos, TV, films, plays, newspapers, magazines and even telephone lines (087..no’s) has become a common trend.
Hence many men and women indulge in fantasizing about others during sexual intercourse with their own partners. This is totally haram and a grave sin in Islam. It ressembles zina and could veritably be termed the zina of the heart or mind!
This point in clearly verified from a Hadith of Rasulullah (Sallallahu-Alayhi-Wasallam) that reads:” To gaze at a strange (ghair mahram) woman is zina (adultery) of the eyes,
To listen to passion stirring words is zina of the ears,
To converse with a strange woman (and derive pleasure there from) is zina of the tongue, To touch a strange women is zina to the hands,
To walk towards her is zina of the feet,
The heart desires and craves;
The sexual organ then either testifies to these or denies them”. –Muslim
N.B. The “desire and craving of the heart” as mentioned in this Hadith refers to fantasizing.