Y’know- you’re right- The Summer of Love was 1967-- right?
Plus- Disco really started around 1974.
Simply changing your name doesn’t mean you can’t get married.
For purposes of his marriage, and all his non-fraudulent activities, he is Don Draper.
He’s only “not really” Don Draper with respect to any fraud he has committed through his adoption of the name – which basically amounts to his act of desertion from the army. So far as I know, he hasn’t fraudulently obtained any of the real Don Draper’s property or other privileges by adopting his name. It’s not like he took over Draper’s credit card accounts, mortgages, assets, etc. Not that most people would have a whole lot of assets like that in 1952.