Sweden: CG married a commoner - all three children have/had relations with commoners who´s backgrounds are problematic and are widely discussed right now.
Great Britain: QEII married a Prince of Greece - their eldest son and heir married a commoner whose background is problematic and widely discussed on this forum.
Sweden: Princess Birgitta & Prince Johan Georg of Hohenzollern - two royals, oldest son has child out of wedlock, daughter divorced and remarried.
I prefer those little noble houses like Wuerttemberg where the children are at least urged to possibly marry another noble person. I think it´s a matter of education. If you tell your child from all the beginning that it´s noble and none other than another noble person be a possibility to marry and if you make sure that your child only moves inside noble circles than the child doesn´t IMO come to the idea to first of all look around under commoners. Well, this attitude might be discussable but it still works in some noble houses and I like that and I don´t think those children feel under pressure or so because lots of them still marry indeed other nobles. (Marie-Christine Habsburg f.i. married into Limburg-Stirum and I´m sure all her brothers and sisters will, like their parents did, the same with all the children of Phillippe and Mathilde of Belgium I´m quite sure)
Look at CP right now with that Sofia Hellquist: I sometimes wonder how all this could have happened though we all know that Silvia is such a 150% Queen - the same with Madeleine - isn´t Silvia the person we see in her - and wasn´t Gracia the person either?
One thing to consider about Grace is that the image of the virginal princess we saw on her wedding day was FAR from the truth about how she lived her life - having many affairs, some with married men throughout her life before becoming Princess of Monaco. Although I think she remains one of the most beautiful princesses ever, the example she set for her children was less than dignified if they knew anything at all about her life. In addition, none of us was there when she was alive. Perhaps she was a completely different person in private, the exact opposite of the regal, dignified persona she portrayed in the media. She was, after all, an actress.
And just a few royal-royal relationships to consider that perhaps did not turn out as well:
Romania - Carol II & Princess Helen of Greece
Greece - George II & Princess Elisabeth of Romania
Belgium - Leopold II & Archduchess Marie Henrietta of Habsburg
France - Henri, current Count of Paris & Duchess Marie Therese of Wurttemberg
Hesse - Moritz, current Landgrave of Hesse & Princess Tatiana of Sayn-Wittgenstein-Berleburg
Iran - Mohammed Reza Pahlavi & Princess Fawzia of Egypt
On the flip side, some of the greatest royal love stories that have existed involved a royal marrying a commoner:
Great Britain - George VI and Lady Elizabeth Bowes-Lyon
Japan - Emperor Akihito & Michiko Shoda
Sweden - Prince Bertil & Lilian Davies
Again, it all comes back to individual responsibility, dignity, and respect. Even with parents who provide a great example, the children can sometimes do as they wish, appropriate or not. Royal families have always had "crazy" people in them, have always had children born out of wedlock (yes even to princesses), and have always had infidelity. Years ago, we weren't privy to such knowledge because it was well hidden and technology such as the internet didn't exist.
But when you look at the PERSONS rather than the title, I believe most people would pick a Silvia Sommerlath, Sonja Haraldsen, or Elizabeth Bowes-Lyon over a Princess Caroline, Princess Stephanie, or Princess Ragnhild.