One of the themes Jane Austen presents throughout the novel deals with the social issue surrounding the middle class marrying into the noble class in order to increase their wealth and social status. The above quote perfectly portrays this social theme through Austen's use of characteristic irony.
It was socially believed, as we witness through the behavior of Mrs. Bennet, that all men with substantial fortunes who enter the neighborhood should become married to one of the neighborhood's daughters. The irony in Austen's opening line, "It is a truth universally acknowledged, that a single man in possession of a good fortune must be in want of a wife," is that, in actuality, it was the women in the neighborhood who were in want of rich husbands and not necessarily the rich me who were in want of wives. Hence, through irony, this line portrays the social belief that all wealthy men were fair game for marriage. But, through irony, it also portrays the theme relating to class intermarriages because it depicts the social truth that middle class women, like Elizabeth, frequently tried to marry men in the noble class. Although Darcy is an untitled gentleman, he is descended of many noble connections, and therefore socially higher than Elizabeth.