My FIL did this to me a few times. It caused rows between me and DH. FIL would always do it at a crowded family occasion, and knew neither me or DH would create a scene and ruin an event.
But he did it once so hard, and in front of all his male work friends who were smirking, I felt so belittled and embarrassed. I cried when I got home. DH was so angry too, but neither of us said anything and we had a row - DH thought it was my responsibility to shout at FIL, but I thought it was DHs responsibility to stand up to him on my behalf. It was his dad and why was he sitting there watching it? But DH said "well why are you just allowing it to happen without saying anything"
I explained that when you're a small woman, surrounded by men and one of them smacks you about the arse while the rest smirk, the feeling of shock, helplessness, embarrassment leave you feeling like you just want to run off and hide.
For further context on this, while I generally get along with all of DHs family, they all think the sun shines out of DHs arse and he can do no wrong. If there's a difference of opinion, or a disagreement between DH and I, they'd always side with DH. And DH knows it and finds it hilarious. He often laughs how he could murder me in cold blood for no reason, but his family would still find a way to blame me for it and help him cover it up.
They also don't like what a 'militant feminazi' I am, and think I'm too oversensitive to sexism, seenit where it doesnt exist and MIL thinks FILs bum smacks perpetrated on other women are 'just the way he is, it's harmless'.
She also loves to cook and host parties, bbqs and any sort of get togethers. So if I was to cause tension at one of her parties because of something she believes is harmless, I'd be all the bitches under the sun. It needed someone who was a family insider to say something, not the wicked DIL looking for a reason to cry about feminist issues.
DH eventually agreed. It needed another man within the family to tell this other man that what he was doing was bloody well bang out of order, for a whole multitude of reasons, and it needed to stop.
So finally, DH had the talk with his DF and its stopped. Never happened again.
MIL told several family members though, complaining about my feminist oversensitivities and crying to the extent another female family member called my DH to say how we'd upset MIL and we were out of line. When DH said "OK, so if your DH went around smacking the arses of other women at every crowded occasion for all to see, are you saying you wouldn't give him both barrels...it wouldnt bother you at all?!"
When every female family member who sided with MIL was confronted with the same question, (all of who had put up with years of being smacked on their arse by him too) they all had to retreat and eat their words. Because my FIL is the only one who did this. Why were all of us just allowing one man to go around smacking womens arses?
Simply because we were all excusing it with "well, that's just the way he is, isnt it"
Well, no. Times change. And its not the way he should be.
Eventually, we had 2 other female relatives call to thank us for being the first ones to finally put a stop to this. Apparently, it had been causing rows in their relationships too but they didn't want to be the ones to say anything and cause friction.
But someone has to, otherwise it becomes a never ending cycle.