NiceGuy....that was a good one! I was working on myself and getting myself together and looking good when when he did notice me and we did get together.
Am I correct in assuming that "we did get together" means you had sex? If you've let him back into your heart (and your bed) this easily before, unfortunately you've kind of taught him that it's okay to cheat on you, and he can always come running back to you and you'll be eager to accept him back easily.
Do the first part -- look good/smell good/act mysterious -- but then hold strong on the second part, and when he wants back -- (((ahem!))) -- "in," tell him "I'm afraid it's not that easy anymore. I respect myself too much to sleep with a man who's sleeping with someone else." (or something similar)
What are your personal boundaries? Have you given that some thought? What are you willing to risk in order to enforce healthy boundaries in your life and in your marriage?