The incident you are refering to occurs in Chapter Four of this great novel by Edith Wharton. Going home to his house, and looking forward to an evening spent just with Mattie, as his wife is away, Ethan is contemplating his situation in life. He surprises Ned Hale and Ruth Varnum kissing as he returns, and we are told that this brings bittersweet feelings to Ethan, as this was the precise place where he and Mattie stood together longing for each other:
It pleased Ethan to have surprised a pair of lovers on the spot where he and Mattie had stood with such a thirst for each other in their hearts; but he felt a pang at the thought that these two need not hide their happiness.
Wharton thus compares the two sets of lovers, and the way that Ethan and Mattie must conceal their love because it is, by necessity, illicit due to Ethan's marriage. As he proceeds, he passes the graveyard and sees the gravestone of his parents, who were together for fifty years. What saddens him is the thought that he and Zeena might have exactly the same gravestone to commemorate their 'happy' marriage:
Then, with a sudden dart of irony, he wondered if, when their turn came, the same epitaph would be written over him and Zeena.
Thus both sights that Ethan sees serve to reinforce the impossible nature of his relationship with Mattie and his longing for her, and all the obstacles that stand between him and happiness.