I KNEW THERE WERE GOING TO BE PROBLEMS...

In those days, "G" actually meant "General Audiences" instead of automatically being a movie just for kids. Grown-ups are part of the general viewing audience, after all.
Hence you have the original Planet of the Apes rated G with partial nudity and violence that's definitely not for kids, as well as the Andromeda Strain which also had partial nudity and some disturbing medical stuff, also rated G.