Hakimul Ummah, Hazrat Maulana Ashraf Ali Thanwi (rahimahullah) said:
In our day and age, audacity and boldness in committng sins has become widespread, which is a very dangerous thing. There are some sins which people are involved in more than others. People consider those sins as insignificant. One of those detrimental sins is lustful glances. This is such a sin, that let alone the general layperson, even the elite are involved in it. They are involved in this sin in spite of the fact that they know it is extremely detrimental and damaging to the imaan and eventually leads to obscenity and shamelessness.
It has been mentioned that a person once saw a pious man in a dream after his death, so (in the dream) he asks that pious man as to how Allah Ta`ala dealt with him. He replied, Allah Ta`ala told me, 'whatever sins you admit to, I shall forgive...' So I started to admit to all my sins except for one (which I concealed) because it was very shameful for me to admit that to my Lord.
For this reason, that sin has not been pardoned from me. That sin is: one time I glanced lustfully at a beardless young lad, and to admit such a shameful act before my Lord is extremely difficult for me. Actually, this punishment which is being meted out to me is lighter than the shame and embarassment I would feel in admitting such an obscene thing to my Lord."
Truly this action of lustful glances is such an evil act that the scholars have written that two sins are such that they are destroyers of the heart. In other words, they completely deprive the heart of spirituality.
One of them is LUSTFUL GLANCES and the other one is GHEEBAH (backbiting), which has become like "milk and sugar" amongst people nowadays." (Malfuzaat-e-Hakimul Ummah; Vol. 4, Pg. 69-70... MALFUZ #67)
[In other words, just as when sugar mixes with milk they become inseparable from each other, similarly these two destructive diseases have become inseparable from the everyday lives of people, and or people have become so addicted to these sins, that it is as enjoyable to them as is milk and sugar. May Allah Ta`ala grant us all protection from His disobedience.]http://blog.muslim1st.com/index.php?option=com_content&view=article&id=122:two-destroyers-of-the-heart&catid=51:islamic-advices&Itemid=53