With all due respect I do not think the marriages of the Prince of Wales or the King and Queen of Spain failed because they were aristocratic. I will say that Queen Sofia, whatever King Juan Carlos may have done, has shown a tremendous dedication to duty and that comes, to a large degree, from her background. Whatever personal unhappiness she may have she has not let it affect her sense of duty to the people of Spain. Something that can not be said for Lady Diana Spencer.
.
Please read carefully what I say! I NEVER stated their marriages failed because they were royals or aristocratic. It was just a remark about Duc et Pair´s claim that a "good marriage" had somethin´ to do with being married "equally". That statement is something I strongly doubt as we have seen many "good marriages" in the last half century between a royal and a born commoner ( I could name you a list, starting with Astrid from Norway and her husband, whose marriage lasts for more than 50 years by now up to princely couple´s from today who are still happily and affectionately married after more than 10 years or so). Of course Queen Sofia was a great Queen, and I admire her. But her royal background didn´t save her from being married to a man who betrayed her for years and did other things I wouldn´t call "the royal way"... This, however, didn´t help the spanish monarchy in the perception of the public.
The same goes for Constantine and Anne-Marie of Greece: Both have a very "royal" background, a young, beautiful and "blue-blooded" couple and still they got kicked out of their country 3 years after being got married.
Of course this could happen to a Queen Victoria of Sweden and Prince Consort Daniel, too! But certainly NOT because she married a former fitness trainer, but because people might get sick of the monarchy, change of zeitgeist, royal scandals, whatever...
Many people have stated quite correctly that Queen Silvia, whose impeccable behaviour and manners should be a role model to many aristocratic or royal born ladies, didn´t put the swedish monarchy into danger because she wasn´t born as a princess, but she might in fact have saved it!
What I say is that Royals can marry Royals if they wish to, and that´s great! But there cannot be a "must" or "should" nowadays! The success of a monarchy these days depends on functional marriages, approach to people and behaviour of the acting royal figures, not their social background (people want to see real people with problems they can associate with, not iconic marble figures put on a dais!).