I am not aware of any statistics on that specific issue. As an attorney, I would typically only become involved if a divorce action were filed. If the couple agrees that no divorce is necessary, there would not be a need for an attorney. As such, I would only be aware of part of the equation, that being those that file a formal divorce action.
With that in mind, I would suggest that included some specific language on the issue in the agreement would resolve the issue for you. If you prefer to avoid the formal divorce action, you might consider including language in the agreement explicitly stating that the parties do not intend to marry. With that language, it should eliminate any belief of an informal marriage and, as such, any need for a divorce.
I hope that answers the question for you. If not, please let me know.
Also, several customers have asked how they may direct a question to me in particular. If you specifically want me to assist you in your legal matter, just put "FOR JOSEPH" in the subject line and I will gladly pick up the question as soon as I am on-line.