Taken from an article on www.jewishsexuality.com
It is important to understand that the damage caused by looking at forbidden images is not to the transgressor alone. The eyes are windows to the soul, and the soul is connected to all of the highest spiritual worlds. Thus, a person who watches pornography on the Internet not only damages his own spiritual and physical being, but also the spiritual channels that bring blessing into the world.
The impurity caused by his transgression pollutes the Holy of Holies of the Temple on high and adds to the prolongation of the exile, the suffering of the Shechinah [Divine Presence], the continued destruction of the earthly Beit HaMikdash [Temple], and the delay in Mashiach's [Messiah's] coming.
On an individual level, looking at impure images creates impure thoughts, and these thoughts pollute a person's faculty of "Da'at," which is seated in the brain. In a sense, a person addicted to por-nography on the Internet "loses his head."
This negatively influences his process of thinking and can cause confusion and other physical problems. The forbidden images he watches also bring about an unholy sexual arousal, and this blemishes the spiritual channel of the "Yesod," something which can negatively affect not only the person, but his offspring as well.