I don't think you're a bad person. I only think that you're very greedy. Too greedy that you want to eat the whole cake and keep it whole. Which isn't possible in reality. Of course your boyfriend will be upset if he'll know what you're doing. This may also ruin the perfect relationship you have with him forever. Do you really want to sacrifice your relationship just for minor temporary pleasures of ego boost?
Ego boost is good to some degree. All women like to feel beautiful and attractive, so it's nice to sometimes being hit on and such, even if you're inside a relationship. But you must keep it at there, don't flirt back, tell them straight on "sorry, I have a boyfriend". Because if you will still continue on engaging in a conversation with a guy that you obviously know that he wants to be more than just friends with you -I personally think that you're playing with fire and disrespect your boyfriend in one of the most horrible ways a girlfriend can.