Some people in Aus consider it snobby to sit in the back if you are one person.. as in, a class thing. There is a cultural pride in everyone at least having the appearance of equality.
Yes, I agree with that. I would be worried about offending a driver if I was alone and got in the back and I guess for a male passenger that belief would be even stronger.
The only time I can remember getting in the back seat when I was not accompanied by another human passenger is when I had a cat in a cat carrier.