I read a stat couple years ago that most men begin exercise because their wives start to. Operative word: most. The other caveat: for a lot of people: weight loss gets harder, not easier, as the weight loss progresses. That's a huge psychological barrier to do more work and see less results.
Alternatively: men only fitness group. My "new" church has one. I've not been, but I understand it's quite popular, among the men AND the women (for some reason or other).