Sex addiction is very, very real. I even wrote a hub regarding it. You are usually set up for it early in your childhood. In my case it was from alcoholic parents whom neglected me and abused me.
What it usually has to do with is, jumping from bed to bed or stimulating the "dopamine and endorphines" in the brain to stimulate " good feelings". It is the means to " reinforce" feelings of need. ie: The more men or women I can get, the better I feel about myself.
This is a very poor explanation in a very short format. It is actually a very complicated subject...
But yes, it is very, very real. It can be very devastating to one's life and believe me, the one suffering does not enjoy it. It destroys their lives and they know it ! There are many groups for it, no different than that of AA or AL-ANON. The victim needs to get in therapy. It will destory them if not dealt with.