grit_fire , I found the solution to this after much head breaking , here concentrate on the word "reference plane " , Ports having same reference plane means that they either have the same X co-ordinate or the same Y co-ordinate , You should see only one white line and not two which ADS draws passing through each port , and don't trust what it looks , zoom it as much as you can and there should be only one line and ports 1 co-ordinate should be exactly same .
Let me know if this is not clear.
Regards'
Gaurav
Indian Institute of Science