I don't think sex can be an addiction, rather a sliding scale with some on one end, who are not interested at all and some at the other end, who think about it and want it as often as possible. Most of us sit somewhere along the scale (Although considering this is a sex toy forum, maybe most here sit above the 50% mark)
All are normal. To think otherwise would be to start labelling (Of which we do, with labels from Prude through to slut) the reality is we all just desire at different intensities.
Its all about perspective really. I mean person A could be called a sex addict by partner B because person A keeps on initiating sex often, but person B thinks it is way too much. How ironic then that if person A got with person C, who wanted sex twice as often as person A, it might be that person A becomes the "prude"
If you see what I mean? So I do not think sex addiction exists. I do think feeling ashamed of your desires or drive exists and that others can make you feel that you are abnormal, which causes you to self doubt or believe you are abnormal and think you have an addiction.
At the very least, sex is not an addiction in the way people would view any other addiction. You don't get sick if denied, you don't start sweating and shaking in a corner, contemplating crime just to get your next fix etc.
It is possible to get into a cycle of thinking about it a lot, thinking about thinking about it and questioning yourself and your desires, but unless you literally cannot function without sex, NEED it to feel well and to get through the next day, heck the next hour then I would not class it as an addiction, just a passion :)
In saying all this, I do believe it is possible to become extremely focused on certain aspects of sex, to the detriment of other areas of your life and relationships and I think our life experiences can give us warped realities on things relating to sex, which can affect our sex life one way or the other. It is quite complex!
Hmmm interesting question!