I recently read a classic Mary Balogh (I won't mention which one because I want to discuss the ending, but you can email me [Magdalen (at) MagdalenBraden.com] if you just have to know the title) where there were two things keeping the hero and heroine apart: a rather flimsy confusion about what "in love" means and the fact that he had a commitment to another female.
This is a Regency romance, so the commitment was rather of the "I've spoken to her father about possibly at a later date seeking his permission to approach her on the question of whether she might like to marry me" combined with "my mother and her grandparents are certain it will be a great match." Which is hardly grounds for a breach of contract suit, but that's not the point.
The point is that the young woman in question Has Expectations and if the hero reneges on those Expectations, he will be a cad and she might be heartbroken. So the fact that he's wildly attracted to, and might be in love with, another woman is irrelevant.
I could discuss whether it's ever a good idea to marry a woman when you love another, but that's actually a morally debatable matter and much more subtle than the pretty obvious point I want to make here. But if you're interested in that subtler issue, I recommend Pamela Morsi's The Bikini Car Wash, which has a nice subplot on this topic.
Here's what irked me about the Mary Balogh book -- the hero and heroine couldn't be together until the young woman to whom the hero had an attenuated commitment was safely paired off with her convenient secret love. The whole business was absurd to the point of lunacy: Young Woman just happens to have feelings for her second cousin who just happens to be the Heir Presumptive to her maternal grandfather's title and estate and who just happens to have feelings for Young Woman. Seriously? In any rational scenario, the grandparents would have had these two married off as soon as Young Woman was of age. (Downton Abbey, anyone?)
But because these poor secondary characters have to keep our hero and heroine apart by any means possible, the entire denouement takes FOREVER and strains credulity to a breaking point.
Whew! Glad I got that off my chest.
I don't understand the compulsion for authors to ensure that every "other woman" short of a true villainess is happily paired off by the end of the book.
Betty Neels often let her heroes make the mistake of assuming that they had to marry someone, and that beanpole female wearing an orange lamé jumpsuit that's cut too low to suit her flat bosom seemed just the ticket. These engagements, as well as indicting the hero's intelligence, could only end with the Veronica (the The Uncrushable Jersey Dress name for the generic other woman, as per Archie Comics) happily paired off with some distasteful academic or rich American. (Exceptions were made where a specific Veronica had attempted a felony, such as engineering an accident to befall the heroine...those Veronicas could be banished sans happy ending.)
I doubt authors do this because they love all their female characters equally. In the Mary Balogh, the Young Woman is a bit of a wet blanket, mopey and not fun to be around. Betty Neels' Veronicas are universally unpleasant, either thoughtlessly or purposefully rude to the heroine.
At least in the Mary Balogh romance, the hero has a cultural expectation of being faithful to Young Woman although it wouldn't have been hard for him to suggest to YW's grandfather that now that they'd spent some time together maybe YW would be happier with ... that Heir Apparent standing over there! Betty Neels' heroes are often actually engaged to the Veronica, but modern-day engagements can be broken rather more easily. Still, each of her heroes seems to wait until some other guy shows up to take the Veronica off his hands.
Here's my rhetorical question: Why can't these books conclude with a mildly disappointed Other Woman at the end?
In real life, there's an alchemy to romance that can allow for person A to dump B, a perfectly lovely person, for C, a similarly lovely person simply because the A+C romance is "better" than the A+B romance was. We feel for B, who did nothing wrong, and maybe it even sours us to the resulting A+C romance. (Woody Allen, anyone?)
But in romance novels with a Hero-Heroine-Veronica triangle there's always a contrast between the nicer, funnier, more compassionate Heroine and the Veronica. As readers, we see why Hero prefers Heroine; it's really no contest. Our sympathies aren't engaged by the Veronica, not the way they are by the Heroine.
Plus, doesn't this equation: A-B
⇒A+C rather insult the intelligence, grace, and independence of B? Balogh's Young Woman wasn't stupid. She could have sorted her own life out on her own, and as no official announcement had been made, she didn't even have to pretend she was the one who ended the not-quite-an-engagement. Neels' Veronicas are so unpleasant that we really don't care about them, but as they managed to get their hooks into the hero, they can presumably replicate that result with some other guy.
Finally, the aspect of these triangular plots that I find the most distasteful is the lengths to which the hero will go to honor a bad or misguided arrangement with the Other Woman. Mary Balogh's hero seemed perfectly prepared to walk down the aisle with Young Woman -- all because his mother and her grandparents thought it would be a good idea? Lunacy. The Neels heroes -- well, they play their cards close to their vests so who knows for sure but in several books it certainly appears as though the hero might well marry the Veronica just because. In other words, they're willing to martyr themselves.
I hate martyrs.