It is normal to get a thrill out of it, because you feel kind of superior that you can grab the attention of someone taken. Trust me, I've done it before and I know what you mean. Even though the feeling is normal, it's not exactly a good idea to do that (as you have already said so). Girls can get jealous and angry and all that, making things really ugly. You don't want to get yourself into that mess.
I know that you said that those girls would do the same thing to someone you are dating, but that doesn't mean you have to stoop down to that level. If you fight them using the same ways that they are using, it's not going to do you any good. I know you said that you don't mean to hurt them, but doesn't mean that it didn't. Everyone can't always agree upon your intentions. And believe me, you won't get a kick out of that.
Plus if those guys are looking at your way when they already have a girlfriend, you probably shouldn't be hanging around those guys. It is normal for guys to look at other women some of the time, but if they keep on doing it, those guys aren't good people. They're just wasting your time when you can be flirting with more decent people who aren't taken already.