The letter, written in 1758 when George and Martha were formally engaged, but not yer married, begins with the following statement by the future president: “I profess myself a votary of love. I acknowledge that a lady is in the case and further I confess that this lady is known to you.” From the context of the letter, it soon becomes clear that the lady Washington is referring to is Ms. Fairfax. He writes: “misconstrue not my meaning; doubt it not nor expose it. The world has no business to know the object of my love declared in this manner to you when I want to conceal it. One thing above all things in this world I wish to know, and only one person of your acquaintance can solve me that.”
But at the time that Washington sent this letter, Sally Fairfax was already married and her husband was a close friend of Washington's, George William Fairfax. He was a planter and member of the landed gentry in colonial Virginia. Fairfax was a good friend of and mentor to George Washington, and he created opportunities for the younger Washington through his powerful family. When the Fairfaxes returned to England in 1773, Washington managed the sale of their property in Virginia.
Sally Fairfax replied to Washington's letter, but her reply was deliberately non-committal. If she and Washington discussed the matter again, there doesn't seem to be any record of it. There is no credible evidence of an actual affair between the two.
When Washington married the wealthy Martha Dandridge Custis, this heightened his social status, and their marriage appears to have been a strong one. George William and Sally Fairfax were the most frequent visitors to Mount Vernon, so if the spouses had any notion of previous indiscretions between George and Sally, it was likely ignored.
In 1773 when the Fairfaxes returned to England. George William was a Loyalist, and the success of the Revolution prevented his return to Virginia. He died in 1787 and Sally then lived alone in Bath until her death in 1811. Washington later confessed to Sally in a letter that she was the passion of his youth and told her that he had "never been able to eradicate from my mind those happy moments, the happiest in my life, which I have enjoyed in your company."
This appears to be the only suggestion of infidelity on Washington's part, though in 1775, the British made an attempt to discredit Washington with a forged letter revealing his affair with “Kate the washerwoman’s daughter.” This was false wartime propaganda and there is no convincing evidence that he was unfaithful to Martha after their marriage.