I have little time left this morning, but this insight seems important, so I shall sketch it out.
Ennis flatly states "I never eat soup." What other unequivocal categorical statements can you think of that Ennis might make at that juncture in time, which also later prove to be wrong.
His denial of soup-eating is met with a wry smile, and the statement, "It is too early in the summer to be sick of beans."
How can we understand this statement in it's explicit context of soup?
If "eating soup" is really a euphemism for something else that Ennis is certain he never does, then, we begin to understand some things. Only towards the end of the summer does one return to civilization and a more normal sexual diet. But, early in the summer, alone on the mountain, with only another male for companionship, soup and beans are really all that is on the menu.