That's another interesting point. Don't get me wrong: I'm all in favour of using sex, violence, bad language, etc if it is dramatically correct to do so, but there is no reason a U certificate film (the UK equivalent of G) cannot be every bit as incisive, challenging, and grown-up as any other.
Great, determinedly grown-up films with a U certificate in the UK off the top of my head:
Citizen Kane
Casablanca
Kind Hearts and Coronets
12 Angry Men
The Searchers
High Noon
A Matter of Life and Death
The Red Shoes
Black Narcissus
2001: A Space Odyssey
Babette's Feast
The Remains of the Day
The Illusionist
Love & Friendship
I could go on...