opb:
That's right, in Fanny Hill the spanking, or rather birching, is regarded as a strange unusual peccadillo of one customer rather than the universal desire we all know it to be. I don't know what John Cleland was thinking.
I did like, however, the phrase "...had to have his pleasure beaten into him as schoolboys do their learning.".
It seemed that there was sympathy for this poor fellow because of his weird need. As it is, I think that Fanny herself made a fairly poor flaggelatrix, mostly because her heart wasn't in it. One needs to know that the person on the other end of the birch is having a good time even if one's own bum isn't
It's been decades since I've read
Memoirs of a Woman of Pleasure, however I did predictably reread the single spanking scene (both the F/M and M/F parts) on numerous occasions back then.
Based on my recollection, Fanny Hill did quite a solid job of whipping her male client's naked buttocks, even though she seemingly had no spankophile tendencies herself and couldn't personally identify with his desire for such treatment--although she did appear to initially enjoy her own bare-bottom switching, albeit only while it started out mildly. After all, she earned her living by catering to the sexual needs/wishes of other people, she knew how to do that even if she didn't share their erotic feelings. Based on the book's conclusion, Fanny's "heart wasn't in it" regarding her situation in general, yet she performed at least adequately in providing various sexual services to her customers.
It may truly have first seemed like a "strange unusual peccadillo" to the young lady, but she had a working-class background while her clients were from the British upper and upper-middle classes. To her, mid-18th-century ass-thrashings presumably were considered pure punishments dished out to children, servants of the wealthy and possibly wives, while the 'English vice' of flagellation among consenting adults likely was largely limited to the upper classes.
Since her narrative apparently was describing representative types of erotic activities which she engaged in during her career as a prostitute, it's also quite possible that she was eventually engaged in other acts of corporal chastisement (giving and/or receiving) which she didn't mention in her account, because doing so would be repetitious.
Your reference to spankophile tendencies as "the universal desire we all know it to be" was obviously 'tongue in cheek,' so in reality John Cleland pretty much got it right--at least from Fanny's initial perspective as a working-class young woman, one who almost certainly had no positive feelings about her own girlhood butt-whippings...

--C.K.