I can see guys using bags, but what about women?
The curved cup-shaped top can be pressed against the body to catch female urination without leakage. Modesty is still an issue. Lookup "TravelJohn" on the Internet.
My daughter (18y/o now) has field-tested several devices for doing her business while off-roading. She rated TravelJohn the best of all, but she still prefers to just go off and "cop a squat."