Where did you get the information you base your argument on? From what I have read, the Mexican government considers all children born on Mexican territory to be citizens regardless of their parents’ nationality.http://en.wikipedia.org/wiki/Mexican_nationality_law#Nationality_by_birth
You can not read “today’s laws” as if they were in place when George Romney was born. They WERE NOT. When George Romney was born, Mexico strictly adhered to natural law & the laws of nations of citizenship at birth via being born to citizen parents whether in Mexico or abroad.
Wiki is good for many things, but it lacks true historical reference when the historical reference does not suit its agenda.