Really, what is it with the term normal? One definition: conforming to the standard or common type. I don't fit that and wouldn't want anyone that does. The bag didn't make me "abnormal" though; I was on that path a very long time ago. And I'm proud of it! I do get a bit upset when it seems that someone is referring to themselves as not normal because they have this bag on their belly.
Don't tell them. Unless your goal on the first date is to have sex, there's no reason to disclose anything. Even then I'm not so sure it's necessary to tell. You just pointed out it's possible to date. The difficulty seems to be keeping a girl around. I think you should not concentrate so much on the telling of what you are afraid to tell...but more so on the personality of the girl you are dating AND if she is the one you think you want to spend the rest of your life with. You will know if she's the one in time and you will feel comfortable telling her all your deep, dark secrets.
If it's just sex you're talking about. You being a guy makes it so much easier to hide that thing. Plus, there is a lot on this topic (hiding or camouflaging the bag) that will help. There was one woman on this site that claimed she had sex with a guy and he never knew she had a bag!!! My opinion on dating lacks a lot to be desired. But it is supposedly just dating...you're not making a life-long commitment. So, don't tell unless the relationship becomes serious. First make sure she is on the same page as you as to where the relationship is heading.