I could not and would not abandon my Roman Catholic faith to marry...but then again I am not a would be Royal/Princely fiancee about to marry into an historic family with centuries of history and tradition linking it to a particular religion.
Charlene has lived in the Principality for several years. She has had time to learn and absorb the fact that not only is Monaco a Catholic country with traditionally close ties to the Vatican, I think the Constitution requires that it's sovereign be married to a person of the Catholic religion.
The chances of Albert marrying in a civil ceremony without the blessings of the Church are slim and none for several reasons...1) He would be the first ruler on the history of the Principality to do this..and it would likely not sit well with his subjects 2)The Holy See does NOT recognize civil marriages without Church blessings as being valid for Catholics, which means that 3) Albert and Charlene's children would be considered illegitimate and would have no succession rights under the Constitution. Remember Princess Caroline's long struggle to have her marriage to Junot annulled and her marriage to Stefano Casiraghi recognzed by the Catholic Church?
It's because she wanted her three children legitimized(sadly, this only took place after Stefano died, and John Paul II annulled her first marriage)