tyrport:
I wrote a story for the newsgroup SSS, Willow Weeps in which I placed exact quotes from the show to set the time, place and set up the joke. They had to be removed for copyright reasons when posted here. I don't know that people got the joke like when I posted to the newsgroup.
At least Willow is a subby character. Hard to imagine Buffy as a sub.
There used to be a set of X-Men fan fiction stories where Wolverine, because of his healing powers was the sub. It's hard to imagine a less subby person.
Well, if a pretty much 'toppish' character is nonconsensually spanked by a considerably stronger one, that might strike me as credible.
I once wrote a "Buffy the Vampire Slayer/Smallville" crossover-style fanfic account, not using the four involved characters' names, in which Clark (Kent) gave Buffy a spanking, then--after Willow had temporarily removed his powers via a spell--she 'returned the favor' to him. Neither of them, Buffy nor Clark, struck me as 'bottomish' personalities *per se,* but each was walloped by a person who was (at the time) physically able to do so forcibly.
Wolverine voluntarily agreeing to accept corporal correction, I would consider that to be somewhat far-fetched based on his personality. However, him being nonconsensually chastised by someone like She-Hulk, Thundra, Rogue (comic book version), the female Captain Marvel (formerly Ms. Marvel), etc., that would strike me as believable, assuming that the fanfic story's plotline provided a credible reason for it occurring...

--C.K.