Story where a guy gets tied to a staircase

So I'm after a story I read many years ago. BB couple have some sort of BB or general sex party. They've specifically invited a younger girl, either a colleague or acquaintance, can't remember.

At the end of the evening everyone has left, but the wife asks the girl if she'd like to try BB on the husband. She's reluctant, but still curious.

Wife ties him against the staircase railings, and the girl does some BB on him, ends up enjoying it. Both husband and wife encourage her quite a lot.

I might not have remembered everything clearly, but that's the general story.

Thanks.