this is an interesting discussion.
It was the cold which brought ennis and jack together, but it was also the cold (snow) that brought an early end to their idyllic life on the mountain. The early snow like an omen, announcing that their union was not to be, no matter what.
It was also very fitting that the reunion took place in the summer, they really found a piece of brokeback when they went fishing for a couple of days. (Hence the skinny dipping scene - no clothes, warm and sunny)
But during the final fishing trip the cold was there again, even though they tried to keep it at bay with their camp fire.
It
was cold of course, but it really represents what a struggle it was to keep their relationship from turning cold too.
God, i'm feeling all sad again, writing this
Their love really should have grown old