You might find this blog interesting:
http://hotwive.blogspot.com/
It's about a couple who started doing something similar, although not with an ex-boyfriend. The girl got involved with a guy she was interested in and knew socially; he thought she was cheating on her husband. Eventually that lie came back to haunt them and she ended up fulfilling the lie by cheating on her husband after he had asked her to stop seeing the man. Now, obviously this had something to do with their relationship and their specific circumstances. It is so very similar to your situation thought that I thought you might find it an interesting read. They are very open with their story with the good and the bad.