Thanks asmonet for bringing up that MythBusters episode. I was coming to do just that.
Guys and gals, there’s poop et al bacteria everywhere. It doesn’t matter where you keep your toothbrush – it, like everything else you own and even yourself, will get covered in millions of millions of microbes of origins you don’t even want to imagine. This is why we have soap, and take showers, and wash down kitchen surfaces after cutting meat on them, and wash our hands before using them to move food into our mouths.
Provided you don’t take a hot dog into the bathroom. forcefully rub it all around the dirty nasty areas, and then eat it, there’s no reason why eating inside a bathroom would be any less sanitary than eating in the rest of the disgusting bacteria-ridden world. Sure, there may be some micro poop floating around but there’s micro poop floating around everywhere anyways, enough to make most of the bacteria you encounter in a bathroom negligible in the grand scheme of things.