You might think we're going to make some sort of statement about our favorite bromance here. And yeah, Watson and Holmes act like a married couple sometimes. But marriage as a theme is dealt with mostly in the cases that Holmes and Watson solve. A ton of these cases involve marital problems and we see more than one example of an abusive marriage here (including the Carey's marriage in "Black Peter" and the Brackenstall's marriage in "Abbey Grange"). As Jane Austen wrote in Mansfield Park, marriage is a "maneuvering business." But in the world of Sherlock Holmes it can also be a very violent business. Marriages were extremely binding in the Victorian era, which meant that domestic violence was something that a lot of people just had to deal with; divorce wasn't an option for most. But laws regarding marriage were also changing in the late nineteenth century as women gained increased rights to property and divorce laws were changed. You can read more about the history of marriage in the Victorian era here and here. The cases that Holmes and Watson encounter often make marriage the site of a crime, and these cases offer up some interesting ideas and questions about marriage as a social institution in the 1890s.
The number of abusive marriages shown in these stories has more to do with the need for a crime drama setting than social commentary.
The sheer number of abusive marriages in these stories points to some sort of widespread social problem and offer up some social commentary in response.