Sorry to jump in here, but I've lurked on this thread for a while, as REB's wife sounds a lot like me in some ways. Obviously there could be loads of complex reasons why she acts like this towards you, REB, and you've had some very helpful suggestions here on what her motivations could be, IMO. It's just that, I have also become quite withdrawn from my husband over the last year or so, and my reasons are much more straightforward. Our relationship is crap and it's only the children that keep us together (we have both ackowledged this at separate times). He has changed physically, and I used to think that if he lost a bit of weight I'd feel different, but I know now it's not that - it's just that mentally we are poles apart and I find it impossible to get sexually excited about someone who in every other way feels like a stranger to me. He's a nice guy most of the time, a good father, and all the rest. But the X factor is just not there and hasn;t been for quite a while.
Why haven't I left? Well, I think that's what 2005 holds for us, but tbh up to now it's just been easier to stick with the devil I know, especially since just falling out of love seems a bit of a cop-out for ending a marriage. If my husband was a friend of mine and told me his wife didn't want to sleep with him any more and the relationship was cool to say the least, I'd say Why don't you leave her?, but of course it's not that straightforward. He doesn't want to leave because he's afraid I;ll try and take the kids away; I don't want to leave because I'm afraid of the great unknown. But we both know it can't go on like this much longer.
I know that all sounds really depressing, but I'm just trying to say that perhaps your DW's behaviour has a much more prosaic basis than fear of pregnancy, or anything like that. Could it be that your marriage (in her eyes) has just gone a bit stale and she can't see how to get back to the way things were? If this is the case, maybe some counselling could help? Would she go?