[Mr. Elton] had gone away rejected and mortified–disappointed in a very sanguine hope, after a series of what had appeared to him strong encouragement; and not only losing the right lady, but finding himself debased to the level of a very wrong one. He had gone away deeply offended–he came back engaged to another.
 

Mr. Elton leaves town after Emma rejects his proposal, deeply offended not only by Emma’s refusal but also by the implication that Emma considered Harriet Smith – someone undoubtedly below him on the class hierarchy – a suitable match for him. Class was incredibly important in early 19th-century England, and while it was certainly a scandal to marry outside of your general class status, it would also have been insulting to have someone even suggest that you couple with a person much lower than you, as it would imply that you could do no better. The biggest hit to Mr. Elton’s ego was not Emma’s refusal but Emma’s assumption that Mr. Elton would have been content with marrying Harriet. He rushes to find a wife during his brief stint away from Highbury to prove to Emma and the rest of the town that he is able to match with someone of the correct class status, and does not need to stoop to Harriet Smith.

Miss Harriet Smith may not find offers of marriage flow in so fast, though she is a very pretty girl. Men of sense, whatever you may choose to say, do not want silly wives. Men of family would not be very fond of connecting themselves with a girl of such obscurity.

Mr. Knightley explains to Emma the realities of what a man of status is looking for in a wife. Emma believes that men are looking only for beautiful women, but in their constricted and class-obsessed society, that was not entirely the case. Men needed to make a match that their family and friends would approve of – reputation was paramount. Additionally, many men were looking for women with a large inheritance to help increase their wealth. Mr. Knightley states that, while Harriet is pretty, she isn’t sensible enough to appeal to a man looking for an intelligent wife, and she isn’t wealthy or well-connected enough to appeal to a man looking for a high-status woman. Harriet’s looks are not enough to help her transcend her class.

But what a connection had she been preparing for Mr. Knightley–or for the Churchills–or even for Mr. Elton!--The stain of illegitimacy, unbleached by nobility or wealth, would have been a stain indeed.
 

Emma realizes how foolish she was to believe that a wealthy, well-connected man would marry Harriet. Unfortunately, Harriet’s mysterious, absent parentage – which Emma refers to as illegitimacy – was already enough to dissuade men of Mr. Elton’s status from pursuing Harriet. Had it come out that Harriet was in fact the daughter of a noble or gentleman, she may have had a chance to marry into a higher class despite her lack of present parents, but it’s revealed that Harriet is the daughter of a merchant with no ties to true wealth. Harriet’s class is far too low to make an appropriate match to someone of Mr. Knightley and Frank Churchill’s status, since they are both incredibly wealthy. Even Mr. Elton, who is a clergyman and therefore lower in class status than the Knightleys and Churchills, would have been risking his reputation on a match with Harriet. Emma should have known this was the case, but she was blinded by naive hopes.

My being charming, Harriet, is not quite enough to induce me to marry; I must find other people charming—one other person at least. And I am not only, not going to be married, at present, but have very little intention of ever marrying at all.

Unlike most women of the time, Emma has no practical reason to marry. Emma’s class status allows her the freedom that most women of her time lacked – for women without large sums of money, marriage was a survival necessity. Emma, however, is incredibly wealthy and can count on receiving a sizable inheritance from her father upon his death, so she has no need for a husband to provide her financial security. She enjoys running the household at Hartfield and is generally fulfilled by the time she spends with her father and friends. Additionally, she knows her father would be nervous and lonely without her, and she intends to take care of him for the rest of his life. So, as she tells Harriet, while Emma herself is beautiful and charming, and many men would be interested in her, she is in no rush to accept proposals. The only thing that could convince her to marry is finding another person “charming”, or, in more serious terms, finding a man that she is actually in love with.