It doesn't sound like your friends have any reference points? I'm not sure they take you seriously. Too, they may not know what a sex addict is and are reacting to their imagined idea of what it is. It's a little like if I were to say in this post, "oh, by the way, I think I have two heads". One wouldn't quite know what to do with that information, how that would affect what one has actually experienced of me in the past, what I have to say, etc.

Someone saying they want to sleep with you can just mean they find you attractive and not really have much to do with sex at all so your response, "I'm a sex addict" could seem a little like a non sequitur.