Not all the service men have fat wives wives. But yeah, it's a stereo type for a reason. The normal ones usually shy away from military sponsored family events - like FRG meetings.  To answer your question, though, despite being 'fit' not all of them are that smart or attractive. It's the best they think they can do. Conversely, some women maintain themselves long enough to land a man and then let themselves go entirely. Oh, yes and that sweet housing pay. Extra money just for being married.