This paragraph is deliberately blurred by intense emotion and a subjective viewpoint, but if you look between the lines, what happens is that Emily kisses Charles, and that Charles finds the experience unpleasant. In contrast to Emily's breathless excitement, Charles is "sober," and his "stepping with one foot into a dry and dusty ditch" symbolizes the beginning sterility of his affections in their marriage. Indeed, he returns to Emily the parasol that she has tossed away, symbolically returning her to her own private sheltering ideal rather than any mutual one. "They turned back" refers also to the verge of passion they had approached, perhaps for the one and only time.