I didn't have this exact same experience but I did want to start a hotwife experience after seeing my ex-girlfriend, girlfriend at the time, take a very large dildo, both thick and long, as she was sucking my cock. We were in the 69 position. Her reaction to taking the entire thing and how she very much intensified her cock sucking made me want to see her get her pussy filled by a cock of the same or larger size.
After her and I talked about it and her initially telling me no for about a month or so, she agreed to it but it never happened before we ended up breaking up.