She may also be bored with the same old same old sex - particularly if she's deeply into her desexualizing roles as wife and mother. This really kills women's libido. Most women want to be desired and courted and paid attention to as an individual - not just as your partner in life and the mother of your children who you just happen to sleep in the same bed with.

Edit: This prompted me to write a story of my own. Thanks!

Maybe Her Low Libido is a Sign of Boredom