Scot Patience and his crewmate were leading for much of the race before being overtaken by their Australian rivals near the end.
But they still appeared delighed with silver, somersaulting out of their boat into the water off Weymouth.
Bithell said: "We tried it. We got the better of them at the start and managed to sneak ahead but we were kind of hoping the fleet would be there to hold them up.
"They sailed a fantastic race, we tried to attack them when we could but they held their nerve and they're worthy champions."
Argentina came third to claime the bronze medal.
Later, Britain's Hannah Mills and Saskia Clark took silver in the women's 470 class. New Zealand's Jo Aleh and Olivia Powrie won gold.
Having seen Australia deny Luke Patience and Stuart Bithell top spot in the men's 470 class, the world champions suffered the same fate on the south coast.
Mills and Clark entered the medal race top of the standings alongside Jo Aleh and Olivia Powrie, although it was the New Zealand pair that won the race and took the gold medal as Britain collected their fourth sailing silver of London 2012.