Several catholic ladies marrying protestant or orthodox kings or princes :
Protestant Baden welcomed catholics Stephanie of Beauharnais, Valerie of Habsburg-Tuscany and Marianne of Auersperg-Breunner (?).
Simeon II of Bulgaria married Margarita Gomez-Acebo.
All their children married catholic spouses.
The present Queen of Denmark married the catholic Henri de Montpezat.
Their son Joachim married 2nd Marie Cavallier
Valdemar of Dk married Marie d'Orléans
Their son Aage married an italian countess
Pr. Michael of Kent married Baroness Marie Christinne v.Reibnitz
The Earl of St Andrew and Sylvanna Tomaselli
His brother Lord Nicholas and Paola di Frankopan
Greece: not sure about Tatiana Blatnik's religion
Pr.George and Pss.Marie Bonaparte
Pr.Christoph and Pss.Françoise d'Orléans
Ernest-August Hannover and Caroline of Monaco
His son Christian and Alessandra de Osma
Ludwig III of Hesse-Darmstadt and Mathilde of Bavaria
Paul-Wladimir of Oldenburg and Maria del Pilar Méndez de Vigo y Löwenstein-Wertheim-Rosenberg
Philipp of Hesse and Mafalda of Savoy
Friederich Wilhelm IV of Prussia and Elisabeth of Bavaria
Pr Nicholas Romanov and Css Sveva della Gherardesca
Cr.Pr. Alexander of YU and Maria da Gloria of Orléans-Bragança
Pr. Alexander and both his wives Maria Pia of Savoy and Barbara of Liechtenstein
Examples of Catholic princesses marrying non-catholics are rare, the other way around happened more often usually with the princess having to convert to her husband's faith at some point.
I don't know of any example where a Catholic princess married outside of her religion and got excommunicated.
No, she didn't, as I pointed out.
Several catholic princesses were almost at the point of making unequal marriages (on a religious basis).
Hélène of Orléans, prospective bride of Nicholas II and the duke of Clarence, prevented to change religion by her father, comes to mind.
Or Maria Gabriella of Savoy and the Shah of Iran.
One that really jumped the fence (and had some annoyances with the Pope) was Giovanna of Savoy marrying Boris III of Bulgaria, but she never got to the point of being excommunicated.
So, what examples are there of catholic princesses really being excommunicated for marrying protestant/orthodox princes/kings ?
But she didn't get excommunicated for reaching such an agreement; so one presumes that even if she had sons brought up as Lutherans, she would not have faced such a penalty ...
The situation was a double-edged sword, since she and her husband were extremely anxious to have a son for the grand ducal succession. Female succession under the semi-Salic law isn't allowed as long as there are still dynastic males in the house. As it was, the future Grand Duke Guillaume IV had a half-uncle still living, when all six of his own daughters were born.
Prince Nikolaus (who had married morganatically) eventually predeceased his much older half-brother, Grand Duke Adolphe, by several months. Otherwise, had he survived both his half-brother and half-nephew, he would have succeeded to the throne -- and likely de-morganatized his own marriage, to ensure the succession of his own son, Georg.
So with the Merenborgs breathing down their necks, it was that much more imperative for the hereditary grand ducal couple to produce a son -- notwithstanding the fact that any hypothetical son would have been raised in the Protestant faith.