There's nothing wrong with the poker game idea as a starter, but both to keep down the number of actors, and to go straight into the action (essential if this is explicitly porn), it should have already happened. All you need is a few lines of dialogue between the top and bottom to establish why she is here and why she is submitting to him, and away you go.

As for the wife's transformation from victim to willing slave-slut, I won't debate whether it is realistic (though I could give you some real cases), but it's a favourite BDSM scenario. I certainly love it