So far as they knew, there had never been any quarrel between husband and wife. They had always looked upon them as a very united couple. (Dancing Men.113)
The cliché that appearances can be deceiving definitely applies to all the stories here, and especially to a lot of the marriages we see in these stories. More than any other type of relationship, marriages seem to be especially prone to deceiving appearances and secrets.
"I tell you that my place here has become difficult, owing to the fact that my employer has proposed marriage to me. [...] He took my refusal very seriously, but very gently. You can understand, however, that the situation is a little strained." (Solitary Cyclist.65)
Though Violet Smith isn't married for the bulk of the "Solitary Cyclist" her tangled romantic relationships give us a lot of insight into how engagements and romances worked in the late nineteenth century. Also, we'd like to give Violet props for being the master of the understatement in her letter here.
"There are two very good reasons why she should, under no circumstances, be his wife. In the first place, we are very safe in questioning Mr. Williamson's right to solemnize a marriage."
"I have been ordained," cried the old rascal.
"And also unfrocked."
"But in any case, a forced marriage is no marriage, but is a very serious felony, as you will discover before you have finished." (Solitary Cyclist.114-120)
We kind of wish Holmes and Williamson could have had an entire story devoted to their conversations, which would have been hilarious. Holmes here points out the type of laws that governed marriage in this time period. Though you might think it's obvious that forced marriages would be illegal, this hasn't always been the case historically. The fact that England had laws against such things made it stand out among other nations in the 1890s.
"It is an open secret that the Duke's married life had not been a peaceful one, and the matter had ended in a separation by mutual consent, the Duchess taking up her residence in the south of France." (Priory School.1.21)
It's interesting that the Duke's failed marriage is described as an "open secret." It wouldn't really become a scandal until an actual divorce occurred in this era.
"I answer that it was because I could see his mother's face in his, and that for her dear sake there was no end to my long-suffering. All her pretty ways too - there was not one of them which he could not suggest and bring back to my memory. I could not send him away." (Priory School.2.64)
The Duke's tragic backstory helps to emphasize how much marriage was still a matter of money and social standing, particularly for those of the upper class, in the late Victorian era. Though the Duke loved James's mother, they were never able to get married. And that situation led to a huge mess involving James's illegitimate status. Being an illegitimate kid, or a child born outside of marriage, was a huge problem for a person in this time period.
"In that case, your Grace, since you have yourself stated that any unhappiness in your married life was caused by his presence, I would suggest that you make such amends as you can to the Duchess, and that you try to resume those relations which have been so unhappily interrupted." (Priory School.2.81)
Holmes moonlights as a marriage counselor here, which is ironic given that Holmes himself generally shuns romantic relationships.
[H]e introduced us to a haggard, gray-haired woman, the widow of the murdered man, whose gaunt and deep-lined face, with the furtive look of terror in the depths of her red-rimmed eyes, told of the years of hardship and ill-usage which she had endured. (Black Peter.2.2)
This scene in "Black Peter" gives us a very graphic depiction of the effects of domestic violence, largely thanks to Watson's descriptive phrases and strong imagery.
"Surely you have gone too far?"
"It was a most necessary step. [...] I have walked out with her each evening, and I have talked with her. Good heavens, those talks! However, I have got all I wanted." (Milverton.1.57-58)
Watson and Holmes are discussing his fake engagement in the Milverton case here, as well as the moral implications of the fake engagement. Holmes's comments about the "talks" he had with his fiancée highlight how little patience Holmes has for people, women especially. There's a good reason Holmes never gave up his bachelor status.
"Sir Eustace was a confirmed drunkard. To be with such a man for an hour, is unpleasant. Can you imagine what it means for a sensitive and high-spirited woman to be tied to him for day and night? It is a sacrilege, a crime, a villainy to hold that such a marriage is binding. I say that these monstrous laws of yours will bring a curse upon the land." (Abbey Grange.27)
Mary Frasier goes on a great feminist rant here about how England's marriage laws are awful and how they favor men over women. If a woman was stuck in an abusive marriage she basically had no way to get out of it; men had to issue a divorce for the most part in this time period.
"Surely your wife knew?"
"No, sir. I had said nothing to my wife until I missed the papers this morning."
The Premier nodded approvingly. (Second Stain.18-20)
The Premier definitely comes off as full of it here, since he "approves" of how Hope kept professional secrets from his wife, for security purposes. Granted, this was probably a good move considering he was married to Lady Hilda, but this little scene indicates how marriage was often not really a union between equals in this era, when women were generally seen as "lesser" than men.