Working out with a VR headset on seems like a great way to get the whole thing disgusting. Hope I can put this $3500 face computer through the washing machine.
Unless Apple has truly screwed the pooch, this has been a solved problem. Sometimes, if you are purposely trying to work out with a VR headset, you just wear a headband to catch the sweat. Most headsets also have cheaply replaceable or even machine washable face "gaskets". If the idea of any sweat in your poor device saddens you, just point a fan at yourself while you work out.
Also, since the gaskets just stick on with magnets, I'm sure there will be third-party ones with included fans or whatever available within a couple of weeks.