It is where the infidelity lies. This is the interesting part. When Ernie was thinking of proposing to Ducky, ne must be mindful of his inability to be faithful to a woman.
Predictably, it's rarely the principle, but rather who's ox is being gored. True, VM wasn't "in love" with Ernie or her sisters, but there still exists a fairness or double standard disparity there. After 30 plus years of "understanding" between VM & Kyril & with their futures inexorably tied to one another VM took a militantly unforgiving (lethal) stance against her husband when she had given a pass to Ernie, her sisters & many other friends & relations. To repeat, infidelity was no stranger to VM. Of course, after so many disappointments in VM's life Kyril's betrayal might have been the last straw but even so throwing in the towel was a strech with so much at stake.
The point is, VM was purportedly "highly principled" and this controversy bring into question her sincerity & motives--not to mention, the machinations she deployed during the years she was attempting to negotiate a separation from Ernie that would be credible in the eyes of her relations. i.e., Her public relations campaign to persuade her royal relatives that it was the lack of "understanding" between herself and Ernie that was the key problem--NOT that she had a long standing aversion to her husband & had fallen in love with Kyril. Under those suspect circumstances VM sought to persuade her relatives that she was willing to "try again" with Ernie even going so far as to have another child (miscarried) by him when she might have calulated that her house of cards would inevitably collapse.
Unfortunately, there is scant known about Kyril's infidelty, his point of view on that subject, or the circumstances surrounding his extra marital relationships. The only perspective known is VM's implacable reaction to Kyril. Posters can only conjecture.
Who's ox is being gored is right. Interestingly, Marie Alexandrovich was a stickler on all things Russian monarchy & Greek Orthodox but did not hesitate to precide over VM's illegal marriage to her first cousin in direct defiance of the edict of the Russian emperor.
It's hardly surprising that Marie Alexandrovich had no-end difficulties with her daughters. This favorite child of Alexander 11 set herself up for one domestic calamity after another. On one hand, Marie preached implacable morality and duty, on the other, she spent her life indoctrinating her daughters to have contempt for men while arranging for them a constant round of "amusements"--balls, dances, theatricals, skating parties, pony rides, bicycling, with handsome grand Dukes, officers, even actors in attendance whilst traveling from one glamorous royal court to another, etc. Talk about living vicariously. When Marie's daughter's did fall from grace (SURPRISE!) their Mama made her displeasure known only to make clear she would defend their reputations & make execuses in their defense to the hilt. Ditto she deployed whatever it took to get that job done.
While Marie Alexandrovna never gave her husband, son, or men in general AN INCH, she did not hesitate to make excuses again and again for her daughters after orchestrating for them marriages virtually doomed to failure. The upshot is Marie Alexandovna's daughters grasped early on that their irregular behaviors would skate. The Duchess of Coburg would only break with her daughter "Missy" when the later sided with the allies during World War One.