I'm also in the same boat as the OP and I'm impressed with the discussion. The last one from Offdee is where I am most days. Is there enough positive to off-set the negative? I'm still married, so I guess the answer is yes - my kids are a huge part of that equation. Someone above said something that makes me consider my situation in a different light though. I do believe it will take a shock for my wife to lose weight, the two prominent/likely shocks being a health problem or me leaving. My thought is - if I left (and I make the assumption she'd then lose weight and become healthier),would that actually be better for her in the long run due to better health? My viewpoint of course is all about me and my perspective, so thinking about it from a different angle is interesting.
That's the rub. If you leave, odds are she will lose some weight and you won't be able to reap the benefits. If you stay, she has no motivation to change so will remain unhealthy. Somewhere in the middle there needs to be a realistic shock to the system that makes her see the light while you still stay in the relationship. The best way to go about this is to improve yourself, make yourself more attractive to women and start gaining the attention of other women. When she sees this, she will be threatened to improve to keep up with your new appearance. You don't need to act on anything and continue to be faithful, but becoming the best you can be is the only way to stay in the relationship and hopefully get an improvement in your wife.