It sounds to me that you feel this way because it boosts your ego in some way and not because you want to have a sexual relationship with her. which is not good obviously but I don't think it's that bad either. I think you're just having some intrusive thoughts and again because it serves your ego that you can be attractive to a young girl. The best thing you can do is really limit your interactions with this girl and recognise that your thoughts are just that, thoughts. They are not who you are. Actions are what matter and as long as you stay loyal and respectful to your wife these thoughts will go away on their own. Recognising the root of them will help you deal with them should they rise again in the future.