Why does my wife of 12yrs not want to have a more intimate or sexual relationship with me?

We have 5 children from 11 to 6 years old. Married 12 years. My wife is a dance/aerobic instructor, therefore in great physical shape. I am in good physical shape. I find her attractive and believe she does me. However when it comes time to need or want sexual activity or just simply a hug or snuggle time in bed, She just shows no interest most of the time. Yes in the past she and I have both admittingly had affairs and supposedly got past it? Just need answers......