All European monarchies followed the rules of this "Almanach de Gotha".
That's not entirely accurate. For starters not all European monarchies had strict, formalized rules about equal marriages - some had very formal rules about equal marriages that essentially prevented its members from being able to enter into morganatic marriages while maintaining succession right. Others only had rules about requiring permission for a marriage to take place - that permission typically was only granted if the marriage was equal, but in these countries there were no formal rules requiring the marriage itself to be an equal one.
Furthermore, the Almanach didn't actually create any rules. It recorded information about every reigning and formerly reigning house in Europe. It recorded who was of a reigning house and who was of a mediatized house. The marriage rules happened well before the Almanach came around, as did the mediatization.
Okay.
I definitely already understood the difference between morganatic and equal marriages though.
So how did it grow, from just being Germanic to comprising ALL royal houses? I actually have a copy of the 2013 Almanach.
All houses have long had marriage rules, with preference being given towards other royal houses over "commoners". The Germans didn't make the rules, they just wrote down who was acceptable and who wasn't - and it make sense that it was the Germans who did so, because the German princely states were considerably more complicated post-Holy Roman Empire than other monarchies.
Again - it's really obvious who's "equal" and who isn't in say 18th century France or Great Britain; either you're the King or you're not. It's more complicated in Germany because there wasn't one unified state. So the Germans went and wrote a book that listed who was who.
I definitely understood the whole anti-German sentiment regarding World War 1 and royalty. I feel, though, that there was a lot of xenophobia; (this is what I'd be thinking, if I was a royal living in the time period) if I can't marry someone because I'm German, what does that make the person/people who established these unwritten rules?
WWI, the anti-German sentiments that came with it, and the mass abolishment of many continental European monarchies following WWI and WWII is actually a major part of the large abandoment of equal marriages in Europe; there were fewer reigning families existing, the Germans were the "enemy", and there was more of a rise of the middle class and a push for monarchies to reflect the people that they reigned over.
In Britain, there was never actually any formal rule requiring equal marriages, and you do see some willy-nilly marriages among the English and Scottish monarchs prior to the Hanovers (more so the Scots, but Henry VIII wouldn't have married most of his wives if he'd only married equally). The Hanovers did have to enter into equal marriages to maintain succession rights to
Hanover, but not Britain itself, and George III did get the Marriage Act passed so that he could dictate the marriages of his children.
Most of Queen Victoria's children married German royals, but her eldest son (Edward VII) married a Danish princess, and one of her daughters (Louise) married a British aristocrat. Of the children of Edward VII, only one of them entered into an "equal" marriage by the old rules - Maud married a Danish prince who became the King of Norway (Haakon VII). His eldest daughter (also Louise) married a British aristocrat, his middle daughter (Victoria) never married, and his son... well, George V married Mary of Teck; Mary's father was of the German nobility and was born as the result of a morganatic marriage. That would not have been in line with the rules that other houses were looking at.
