Also if anyone can become a Queen, people may well start asking why can't just anyone be a King. If the royal families start looking middle class, a lot of normal ordinary people can start to say, well, I could do a better job than they do and a lot cheaper.
We see it in a lot of democracies that when politicians elect the Head of State they cose candidates who have good manners and a suitable spouse to support them. So people have come to expect this kind of behaviour from the Head of a State and his/her family. When the daughter of German Bundespräsident Scheel announced in public that she is homosexual and in a relationship with a female TV- and comedy-star, a lot of people were aghast. But at that time her father was already the ex-Bundespraesident, so it didn't matter that much.
So I don't see different standards for Royal families or the families of elected Heads of State when we talk about the public's opinion.
As for the opinion that marrying commoners diminished the luster of the Royals: you need the media to whip up this feeling against a new princess and later her children. So far the media has not done that and I doubt they'll do it as long as the commoner princess behaves like a Royal. As I said before: politicians and the media are very well-aware that a change of the constitution has serious dangers to the stability of a country in tow. Which is something they don't want, the media especially because they know their readers don't want that.
So I don't think the media will ever use Catherine's background against her as long as she and her family behaves properly, as expected. Because in belittling a commoner princess for her background, without any other reason, would mean to belittle their readers.
Thus I think all this talk that commoner blood is weakening the monarchy has no basis in the way the political class or the people think about it.
What can weaken the monarchy is individual behaviour: The War of the Waleses was dangerous because both Diana and Charles did not behave like Royals should and became topics for the sensationalistic media. And there is no proof that today's princesses of the blood will be able to handle a Royal position better than a commoner girl. The times are gone when princesses were raised to do their duties and to behave appropriately, no matter what. We know today form decades of adolescence research how important the peer groups are in forming a character and there is no longer a closed-in Royal world to form the peer group of young princesses. They mix and mingle at school with all kind of girls from rich families: the daughters of drug barons are as well educated at posh schools as the daughters of popstars, self-made-millionaires or of the nobility.
Once the Royals decided to send their children to democratic schools, all things changed. So today you can only rely on the individual character of a person, not on her name or her background. Just take a look at prince Louis of Luxemburg and his wife Tessy de Nassau. Tessy was, compared to Catherine Middleton, a girl from a very ordinary background who was a soldier. There she met the prince, they fell in love and she got pregnant. Her married her only after their son was born, so she certainly was not "princess"-material at all. But ever since she has behaved with great dignity and it is clear whenever she appears at official events with her in-laws that she is well-liked and respected by them as their daughter-in-law. She will never be a princess of Luxembourg but she surely is a good representative of the the old name "de Nassau" and that's what people in Luxembourg like about her.
I have absolutely no problem with morgantic marriages and I think in the case of Edward VIII it would have been a good way out of a very delicate situation, but it seems the British Government does have a problem with this kind of marriage.
I think we get back to the "non-negotiable" part, as I believe that was said by Prince Felipe, the very much loved only son of the sovereigns. As you say after that happening, you never know, it was almost a "if pigs could fly" situation but he pulled it off. Whether Prince William has the same dedication to Kate is the question then.
