Re: Shouldn't we have it all??
Except this isn't improving everything at once. He is addressing one specific topic - her weight gain and it's affect on his physical attraction to her.
And more importantly his level of resentment over the fact she hasn't taken care of the issue herself. Him waiting to say that isn't going to magically clear up the resentment he feels, and expecting someone who resents having to be involved at all to take more of a lead I don't honestly see working.
OP, you wanting her to become a self-motivated gym rat like yourself who wants to work out for the joy of working out, frankly is unlikely to happen. You want her to want to lose weight, and you want to dictate the reason she is doing so. You don't want to get "emotionally tied up in it." You are threatening to divorce over it - you are already emotionally tied up in it.
And I can't speak for the experience between you and your wife, Blue, but I can speak as a woman who has lost a significant amount of weight and then regained some and trying to lose it again. I can speak to the effects a spouses support has on those efforts.