A common trope in Regency romances is the couple who must marry because they were caught in a compromising situation.
Imagine that Elizabeth and Darcy have met again at Pemberley--no longer actively antagonistic but still wary with some of the same family hindrances--but they are caught making out by, say, one of the Bingleys, and the rumors start to fly and to protect Elizabeth's honor, the two must marry.
Basically, Lydia and Wickham blended with Elizabeth and Darcy.
It isn't a completely horrible plot. The very funny The Viscount Who Loves Me by Julia Quinn is partly based on this approach. It can work.
But it suffers from immediately catapulting the plot into "oh, no, we only married because we had to" angst, which can get a tad tiresome. It isn't that the conflict has zero teeth, but it's not clear where it is expected to go. Unless the book becomes all about dysfunctional people who hate each other (which plot can also be rather tedious), the only plot left is for the characters to adjust.
Adjustment plots can offer some substance. With A Civil Contract, Heyer produced a book in which the young high-status hero marries the daughter of a middle class financier (of rather crass manners), all in order to save his estate--and the people who live on his estate. He gives up, he thinks, a great romance with a young woman of his own class who tends to swoon in dramatic moments.
Over a year later after the birth of his first child, he finds that he likes his wife and is far more interested in innovations to his estate than enacting melodramatic scenes with the ex-girlfriend, who is married now to someone else anyway.
Still, one can shake the "Uh, what else were they supposed to do? Hate each other and be miserable? Who wants to read that?" sense of inevitability.
Granted: part of the attraction of romance is HOW a couple works things out. But if one is writing narrative arcs, it is best to preserve some point of change or active choice...rather than to say, "Well, yeah, of course everybody trundled along okay. They are reasonably civilized people, after all." And since the point of change or choice for a "forced marriage" with a HEA (Happily Ever After) is to not have an affair, not commit murder, not wallow in self-pity, not build a separate staircase to one's study (Thomas Hardy), not write blistering letters to each other for public consumption (Bryon and...everybody)...
I'm not sure I see the point. Oh, thank goodness, nobody acted like a total jerk is not the most satisfying pay-off. If, on the other hand, the ultimate point is that the marriage won't work out, unless the book is a murder mystery, a non-fiction tome about the dissolution of modern marriages will achieve the same goal.
Austen understood marriages--she was quite observant. But she was writing narratives, not self-help manuals.
