Your wife had never been fucked like that.
You had rather sheepishly confessed to your wife that you sometimes fantasized about her cuckolding you with your best friend. You weren't sure how she would react, and you certainly thought the idea would just be, at most, a fun fantasy for the two of you to role play. But your wife's reaction was ... enthusiasm for doing it for real. And now, it seems, there is no turning back. It's going to happen.