I just wish I knew how to help. Once us girls get something into our heads, it can be very hard to shake. The only thing I can suggest is giving your wife the same treatment she's giving you. Give her plenty of space. Ignore her. Go out without her and have some fun. Act like you don't care anymore that she wants out. If she is just in the middle of some sort of crisis and truly doesn't want to leave, see how quickly the fear of losing you jolts her back to her old self.
I think this is probably the best advice. A few thoughts:
Women generally check out of a relationship emotionally well before they do physically. So her ambivalence is a sign she's already out of it. It may not have anything to do with you, it could be her dissatisfaction at being a stay at home mom, her own need for the kind of excitement you can't give her after the familiarity of a long term relationship. Who knows. She seems uninterested in fixing things.
Also, the advice to look out for an affair is sound. She's SUPER vulnerable to the attentions of another man right now. If she's not in an affair, she'd be ripe for the picking from a predatory man.
I'd confront her in a gentle way. Something like, "It feels to me that you are checking out of this relationship. I'm going to be making some changes in myself, and I'd like for this to work. I'd like for you to be on board and I think we can create the kind of passionate marriage we both want. Either way I plan on having more fun in my life." Then leave it at that and make it your god d*mn MISSION to have fun. Plan fun things and invite her to come. If she doesn't then you do them anyway.
Oh, and if you discover she's having an affair, then buckle in because it's going to be a brutal ride for you. Sorry to say it.