Wow - you guys are harsh! Guess we've all been there.
At least your wife is being honest with you by letting you know there's no romantic future in the cards. And she IS sticking around for the benefit of the kid(s).
You have to get over your hopes for a happy ending (in either sense) with your wife, and decide between a loveless though semi-functional family situation - and breaking up the family for a more gratifying relationship (ok life).
But bottom line, your wife has already put her personal wants at odds with the needs of the family and it's only a matter of time before things blow apart. So yeah, plan your exit at YOUR convenience, have no guilt, and find someone that DOES want a real relationship.
It'll be hard at first, but it won't be long before you start wishing you'd done it long long ago.