The discussions goes like this: During mastarbation on porn-images/movies the brain gets flooded with dopamine - that never happens with sexual intercourse. The dopamine-flood reduces the number of dopamine receptors (as a protection), so next time you need even more porn to get the same effect and the spiral get worse. The excess levels of dopamine has an effect on the blood vessels in the brain. Some say vasoconstriction some say vasodilution, I dont know which it is, maybe both and excess dopamine also make you loose all motivation and get more susceptible to other forms of addictions (facebook, games etcetera), that increase the dopamine even further. You simple have to stop it cold turkey and the receptors will heal in some months or a year.