I appreciate your taking the time to point out my mistake. I have reconsidered, and I do believe you to be absolutely correct in that the U.S. Constitution does nor forbid a citizen from holding a noble title; it does forbid Congress to grant such titles. Your correction makes sense as there are numerous Americans with titles: Princess Yasmin Khan (daughter of the late actress Rita Hayworth), is but one example.
In one of my numerous books, I did read where HSH Prince Rainier announced that his children were Monegasque citizens only. It's probably a topic that he prefered not present itself, but it did, and he made the proclamation. This was done because, as you point out, the children were born American citizens since their mother was American. It's probably much more of a technicality than a practicality. USA in unique in that one is a citizen simply by being born there or if a parent is a citizen. If other nations had the same law, my American cousins who were born of an American military family stationed in Japan, would be Japanese citizens as well, simply because they were born there.
Not surprisingly, citizenship laws vary from nation to nation. Having lived in México, we were considered Mexican "residents" but were not required to relinguish loyalty to our sovereign (in Britain we're technically "subjects" of Her Majesty, not "citizens" of Britain). This was simply financial for the government of México, as foreigners pay various fees for the process of becoming a Mexican "resident". Few would embrace the residency requirement if if meant relinguishing citizenship or loyalty to the mother country. A smart move on the side of the government, in my opinion.
Thanks again Chuck, for taking the time to point me in a more enlightened direction!