The previous day (to the passage you quote), Emma and Harriet have been discussing the Martins, and Emma expressed her opinion that Mr. Martin will probably (Emma's meaning: surely) marry an uneducated farmer's daughter, and that Harriet should not be acquainted with her. Harriet responds:

"To be sure. Yes. Not that I think Mr. Martin would ever marry any body but what had had some education -- and been very well brought up. However, I do not mean to set up my opinion against your's -- and I am sure I shall not wish for the acquaintance of his wife. I shall always have a great regard for the Miss Martins, especially Elizabeth, and should be very sorry to give them up, for they are quite as well educated as me. But if he marries a very ignorant, vulgar woman, certainly I had better not visit her, if I can help it.". (Ch. 4)

In chapter 7, Emma also tells Harriet that if she had accepted Mr. Martin's proposal she would be confined to a vulgar and illiteral society all her life.