I'm not convinced Pinter must have been a spanko (though I admit it's more likely than not).
smeple:
Pinter makes several of what could be considered quasi-sexual references/double entendres when talking the girl who may have wanted to be spanked.
Yes, you're absolutely right, the language Pinter uses to describe the girls (either "the girl", or "the other girls" he talks about in the next-to-the-last paragraph) is very sexual. It's pretty clear in this piece that girls who "like to be spanked" are turned on by the idea.
But Pinter isn't even sure if "the girl" is "one of those girls" (who like to be, you know, spanked). Maybe she's just doing "honourable research", he says.
And even if she is or not, none of this implies that Pinter has any special interest in spanking himself. Perhaps Pinter has known some girls (maybe some former girlfriends, wives, or mistresses, as from his biography it's clear he had multiples of all of these), who have told him (with breathtaking spasms of candor!) that they like to be spanked, and this piece merely draws on his research into these matters!