I assume they were monogamous before?
I have heard numerous stories of monogamous couples divorcing then remarrying, then divorcing. In traditional relationships if you love someone, you are "supposed" to live together. And yet many people who love one another simply are not good at living together.
My thought when I read this was that perhaps your husband and his ex can enjoy the feelings they have for each other if not confined to the traditional structure. Depending on the nature of the issues that caused them to divorce, maybe dating is not so odd if it was some version of the living configuration that contributed to their issues.