Why do I have desire for the opposite sex being happily married?

I am married and have been for 16 years. I love my husband. We have three children. We have very lucrative jobs. We are active in the community, as well as, with our children, ages, 12, 6, and 3. However, something is missing. I feel the need for more. I do want to have a physical affair, but I do have the desire to talk to other men ----- constantly!!!! What is this?