I'm pretty certain that it's a reference to being out there on one's own, with little or no support.The rented pants thing in Friends could be seen as unfamiliar, possibly hostile territory, whioch continues the image.Def
It is true that many commandos do go without wearing undergarments from time to time, if not more often than not. The thing people don't understand is why. In a few of the sources I read for this fact, actual commandos were asked why they prefer to 'go commando'. What they said is not for those with weak stomachs and vivid imaginations. Sometimes they go commando because they will be walking through rainy areas for many hours / days at a time. In such situations, wet underwear can lead to some disgusting things like crotch rot, and some unfortunate jock and anal itching. Walking for extended periods of time, even in dry climates can lead to some very unpleasant chafing. Extended time in tight underwear causes infertility to top it all off, so many soldiers have sworn off undergarments entirely.On a more disgusting note, some of the soldiers told stories of when food rations were not up to par with health standards, therefore causing insane diarrhea. This, they say, is the true definition of going commando: going without underwear and cutting the back of the pants completely off to give your unsettled stomach contents an emergency escape hatch (for lack of a better analogy).