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the best way to motivate your wife to stay in shape is for you to quietly and without fanfare, get into shape yourself.
She'll see that... wonder what you're up to... and then figure out that she's got to raise her game as well.
And if anyone is 100 pounds overweight, then the loved one in their life has to have a heart-to-heart about the health risks of obesity. At that point the person involved is probably in such a psychological funk over the weight that being approached about the partner's sexual needs can be perceived as judgmental, self-serving and unloving.
But to tell the wife, I'm working out for health because I want to live a long and active life, and I'd like for you to do so as well, would be perceived as being more caring.
The side benefit would be increased libido. A lot of lack of sexual interest is mainly fatigue from the hard work of raising babies. Just ask a high-pressure career couple each working 50-60 hours a week, how much energy they have for sex, and I'd venture their activity has fallen way down.