Without turning this into a monster monarchy thread-the idea that Royals may only get with fellow Royals, thereby promoting inbreeding, is a little odd.
Equally, the classification of everyone who is not a Windsor as a 'commoner', is also rather odd.
That has been common practice for many hundreds of years. Prince Philip and the Queen are cousins remember and at one point, he monarchies from Germany, Russia and Britain were all related (each king was brother to the other if I remember rightly...something like that) and each married off his relatives to the others relatives to conserve the purity of the royal line.
Then add to that the fact that the aristocracy going very far back in time thought it was perfectly acceptable to marry within their families, some fathers even going as far as to get their own daughters pregnant so as to conserve the family bloody line and stop common seed polluting their geenseed.
But then they didn't realise that doing this caused mutations in their children, due to the lack of genetic diversity and was probably part of the reason why a lot of their children sometimes didn't carry to birth. So it wouldn't be surprising if a royal married a royal. I think with Diana, Sarah Ferguson & Kate we've now had more "common" geneseed enter into the royal line. Of course most of them had aristocratic upbringings lol