I think you need to honestly examine your marriage and answer the following questions as truthfully as possible:
-Is she satisfied with the sex/ does she orgasm every time?
-Has she always been satisfied in the past?
-Is she too tired for sex? (Does she work outside the home, take care of the baby and do the lion's share of the house work?)
-Does she receive any attention or affection from you when you don't want sex? (Holding hands, snuggling, kissing good-bye and hello...)
If none of the above is an issue, she may be bored and looking for the excitement of a new relationship. If this is the case there is very little you can do to change her mind. You could suggest role playing or new positions or his and hers lotions... but ultimately,she is the only one who can change things.