I think part of the Megan/Don relationship has to do with the growth of women in the late '60s. Don is learning to cope with a woman who doesn't aspire to be June Cleaver. She is being more assertive and wants her own fulfilling career. Don has issues with all of this, but when Megan holds her ground, Don eventually sees the light. Yes, Don has grown as a person, but some times backsliding is part of that process. As much as I want Don to remain faithful to Megan, I suspect that will not be the case. I mean he still is Don Draper and they still need to make an interesting show.