Don pulled a surprise when he impulsively married Megan after she was his nanny with his kids on that trip to CA or wherever. He had been seeing that more sophisticated blonde consultant who he strangely dumped. It seemed that he picked her mainly because of how she was with his kids (at least at first). She later brought out her "Zoobie Zoobie Zoo" song and dance that mortified him, as well as her acting dreams that were never put on the table before the wedding. Don has been more the upstanding, moral adult in many situations over the last season compared to all the other male characters. They may feel the need to change it up, but I felt that last season was the best one yet in many ways.