Having recently married, I can't tell you if we'll remain monogamous forever. As it stands today, I certainly intend to try. But I think that's a lot of pressure to put on any relationship. Monogamy has to be a lifelong dialogue that takes place as you grow and evolve.
So, you're happily married. Really, truly! But you meet someone who just...pushes your buttons. Ever consider a polyamorous relationship? Opening up your marriage to the possibility of another partner can be very complicated, but rewarding -- as long as the lines of communication stay open, and honesty is the focal point.