You don't have to read much primary source material from the 18th and 19th centuries before you realize people were far more formal in how they addressed one another. I call my doctor Leslie and my mother-in-law Marcia. With the exception of one very old-school professor, I've called every single boss I've ever had by his or her first name. In Pride and Prejudice Mr. and Mrs. Bennett call each other exactly that...and lest you think that's because theirs is far from a model marriage, at the end of Emma the heroine announces her intent to keep calling Mr. Knightley exactly that once they're married. (Of course, this wasn't invariably the case. As far as I can tell, the Duke and Duchess of Wellington consistently addressed one another by first names in their correspondence, despite their marriage not being even remotely happy and intimate.)
So when you read all this, or at least when I read all this, you can't help but wonder whether it applied in the bedroom. Was it "Mr. Bennett" and "Mrs. Bennett" even while they were in the act of conceiving their five daughters? And what about less equal relationships, of gentlemen with courtesans or prostitutes?
Which brings me to 18th century pornography, specifically Fanny Hill. (WARNING: Link contains NSFW images.) A 1748 novel by John Cleland, it purports to be the memoir of a country girl who went to London for work after being left orphaned and spends the next several years as a prostitute and occasionally a mistress before eventually marrying her first love and living happily ever after. The text is readily available for free online, and it's an interesting glimpse into how people of that era thought and wrote about sex. But it's porn, make no mistake. Don't say I didn't warn you.
Anyway, there's one scene where a prostitute, in the arms of her highborn lover, cries out, "Oh, Sir!...Good Sir!...pray do not spare me! ah! ah!..." So, there you have it. If you were in the 18th century, in a sexual relationship with a man who outranked you by a country mile, that intimacy was no excuse to drop the proper formalities, even in the throes of passion.
Now, I seem to be developing a habit of writing cross-class love stories. The Sergeant's Lady has a common sergeant and an aristocratic widow. My current manuscript has a relatively common heroine who, on her way to her happily ever after with someone else two books later, is going to have an aristocratic lover. Are any of these couples going to be formal in bed? In a word, NO. Will and Anna of The Sergeant's Lady aren't on a first name basis till after their first kiss, but well before they become lovers. And my new characters I think are going to use nicknames when they're alone together.
Because, even for those of us who are dedicated to our research and try to make our novels as historically accurate as possible, there are limits. And having my heroine call her lover "sir" in bed is one of mine.