You might as well ask a drunkard why he abuses alcohol even though it destroys his liver. Or a smoker why he keeps smoking even though he's read the warning labels about lung disease.
Studies have shown that the parts of your brain that are triggered by sex are essentially the exact same ones that are stimulated and respond to any other type of addiction. People essentially get addicted to sex, and you can't rationally talk them out of it in the same way you can't rationally talk a smoker out of wanting a cigarette.
This isn't to say that sex is as hard an addiction to break as smoking or drinking, but that it is very possible to become addicted to the feeling in the same way you would become addicted to nicotine or alcohol. If you wish to break out of a cycle like that, you have to approach it in much the same way that you would approach dealing with an addiction.
