"I married Mrs. Mary Burton"

"Mrs." is an abbreviation for "Mistress", the feminine form of "Master" or "Mister".

In the 18th century, Mister and Mistress were used only for those of high birth who would be employers of servants. Mister and Mistress did not indicate the married state at the time. By referring to the woman he married as Mrs. Mary Burton, Gulliver was not implying that she had been divorced or widowed, merely that she was of comfortable social station.