Dr Jekyll and Mr Hyde is my only nineteenth-century British novel that isn't explicitly about marriage, unless Jekyll/Hyde were a sort of queered marriage—in which case it does work as a novel about marriage because, just like the century's other anti-marriage novels, the pair start by making each other miserable and end in a double murder/suicide. Which is essentially, says Hardy, what getting married is about in the first place.
(Helpfully for my theory, The House of the Seven Gables doesn't have much to say about marriage, making it distinctly un-British. Its sense of threat is located around 18th-century-style family-paranoia issues—although it gets over these in the end by making the last descendants of Maule and Pyncheon marry each other—which is nice, I guess? It's a very hopeful novel, anyway.)