I faced same problem like you , 2 times
1 st time :- I used registration same like you, then i have changed my application's flow. made registration as optional. and they accept it .
2 nd time :- I replied to apple that , "please clarify me regarding rejection issue ,sorry, but i can't able to understand why my app rejected ?" I can't do anything and application will be live on 2nd or 3rd day.
So, my opinion is , 1st ask to apple to clarify rejection issue in detail. if your luck then it will be live. or wait for their replay and then modify your application according to their replay.
Edit:- When you upload the app on App Store and your app include "Personal information from users -> Example registration" , try to provide "why you need personal detail" or application's flow in description .