I think is strange no one has mentioned this before. Valmont tells Cecile that he slept with her mother before she was born. Madame de Volange did not sleep in the same room as her soon to be husband, and could therefor visit Valmont in his room that night. She became pregnant that night, and since she did not want Valmont as her husband, she therefor told the other man that he was the father, and they had to get married. Later in the movie, Cecile becomes pregnant with Valmont, but has a miscarriage. That is because the baby was a result of father and daughter incest. Read the book, and watch the movie closely, there are more clues..
Review of Dangerous Liaisons
