Gatorade provides multipurpose bottles for this purpose. Hydration first, then re-used for... umm... un-hydration?
Not a recommended practice when you have both sexes in the plane. Keep it zipped up. 
it's not a preferred practice regardless of who's in the plane; that's not the point. Among adults, there's courtesy, professionalism, and necessity. If/when you're in that situation, everyone else looks somewhere else.
Mission and safety (hydration) are first. Whether I'm in a convoy or on a glacier with three rope teams, both genders just show some professionalism, turn away, and admire the sunrise / crevasse / stars.
Back to CAP... this is nothing that should be done with cadets present, but why would cadets be on a 4+ hour flight?
Gatorade bottles are easier for the men, of course. No argument there that guys have it easy.
Commo