Jackie, the guy who "took it back" is an idiot. Someone who would do something like that is completely ignorant of the situation and not concerned enough to learn, and you don't want to be involved with someone like that. As for Gio, I had really hoped my gender had evolved beyond crap like that, and I can promise not all guys are that horrid. You are not a disease, you are a person, and you deserve respect!
As for the guy you like, there are two options. One, you can keep wondering about whether or not to ask him out, which will slowly but surely drive you completely mad. Two, ask him out and be upfront with him on your date about diabetes. If he is cool with it, you are all set. If he isn't, then you didn't want to date him anyway. The fact is that there is nothing wrong with being ignorant; everyone is ignorant about something until they learn about it. The key is the willingness to learn. If your guy says, "OK, tell me about it" then great! If he asks "Is it contagious?" don't get mad at him, he doesn't know yet. Good luck!