In 1771 Pss Leopoldine of Liechtenstein married Landgrave Karl-Emmanuel of Hesse-Rothenburg.
I guess he would qualify for a reigning chap.
In 1713, Pss Maria Theresia had married Duke Emmanuel Tomaso of Savoy-Carignano, Duke of Soissons, from the cadet branch of the Savoy reigning house.
As to the Alois-Elisabeth Amalia wedding, despite some Habsburgs having raised their eyebrows and considering it a mesalliance, the Emperor himself did not and, to make it clear, honoured the wedding of his niece with his Imperial presence, shutting all mouths.
Was the marriage in 1882 of Prince Arnulf of Bavaria and Princess Theresa of Liechtenstein the first union between the principality and a royal or reigning house? The reason for my asking is that members of the princely house have historically married members of the nobility, not royalty.
Did anybody in the Bavarian royal family say anything against the marriage, calling it a mésalliance? This might have been the situation, if there was no prior history of interrmarriages between the princes of Liechtenstein and sovereign houses.
It's anybody's guess as to just what exactly the bride's father-in-law, Prince Luitpold (who himself had married an Habsburg archduchess of the Tuscan branch, and whose own two eldest sons had likewise married Habsburg archduchesses: the second son, Prince Leopold, even married a daughter of Emperor Franz Joseph of Austria), thought of the match.
But in the end, I suppose it would not have mattered, as he was not yet serving as regent of the kingdom, in which case he would have been the DE FACTO sovereign. As of that year, his nephew (the so-called "Mad" King Ludwig II) was still on the throne, and therefore would have had the final say on approving or disapproving marriages.
To the best of my knowledge, that union was dynastic from the beginning. There is no historical record of it being initially morganatic, but de-morganatized later on. So one presumes that King Ludwig II gave consent to his cousin's marriage ...
Of course, it must have helped that Arnulf (the youngest son of his uncle) had no chance in the world of ever succeeding to the throne of Bavaria as king, anyway. Under that scenario, Theresa would have become the country's queen.
As it was, in the next generation, there was some opposition initially raised against the marriage of her nephew, Prince Alois, and Archduchess Elisabeth Amalia of Austria -- a niece of Emperor Franz Joseph, and first cousin to Archduchess Gisela (who was married to Arnulf's brother Leopold). This, in spite of some Habsburgs having previously wed members of mediatized houses ...