Easily turned on by short skirts and heels, surf fb profiles

I'm 28, and I have a girlfriend, We've been together for about 4 years now. We have plenty of sex when we are together. I have nothing to worry about sex. But I have this fetish of looking up random girls on facebook, go through their pics and find the ones where they r wearing skirts, or exposing their thighs. I make a collection of them and jerk on them. I feel very satisfied while doing this. After I'm done, when I'm back to my normal self I ask myself, Is this normal? Part of me says it's just fantasizing and being imaginative. The other part just tell me ' I m Sick'!!! But is it though?