Seegee:
In BtVS Anya, to Xander's horror, once mentioned that they had tried spanking. I always wondered who spanked who?
IIRC Willow mentioned to Anya and Xander that Buffy had been busy lately because she had "a spanking-new boyfriend," to which Anya replied, "We've enjoyed spanking."
I'd figure that they switched off, taking turns dishing out the paddywhacks.
Anya was a one-time vengeance demon, she'd done terrible things to men who'd mistreated their womenfolk, so I'm certain that (her being more or less reformed from killing) she could've understood the concept of administering less-than-brutal but still moderately hurtful chastisement to Xander--IMHO he certainly ended up deserving it by leaving her alone at the altar later on in the series.
(Okay, I have to take advantage here and note that I did write a 'switchish' short fanfiction story, "Balancing the Books," involving Anya and Xander.)
Later in that same episode of BTVS, Buffy and Willow were watching Buffy's new boyfriend going through the serving line in the school cafeteria when, seeing him choosing some 'empty calorie' selections, Buffy muttered, "He's going to be so punished." Willow's somewhat forlorn response was "Everybody's getting spanked but me."
I'm figuring that Tara eventually took care of that missing aspect of Willow's life... --C.K.