The big one is about cheating. Infidelity on the part of the hero or heroine is an absolute big-time no-no in the romance genre. Now, I understand it's a sensitive topic, and that cheating has hurt lots of actual people, who therefore don't want to read about it. However...yeah, it does happen to lots of actual people. Therefore it's a pertinent issue. And while infidelity is usually not the *best* idea, I wouldn't qualify it as pure evil in most cases either. And, more to the point when we're talking about writing, it usually makes for juicy plot twists. Therefore, though I don't want to include it in all my stories, I do sometimes explore the sticky and interesting issue of being not 100% faithful to one's significant other.
Mind you, in both the published books where I've gone into that territory--What Scotland Taught Me and Relatively Honest--I was dealing with teenagers, not married adults. Age 18 is a time when plenty of us make questionable decisions, and learn from them. I was going more for realistic coming-of-age than strictly for romance. Nonetheless, I think a love story benefits from a dose of reality--and a dose of juicy gossip.
Also: how come we modern romance novelists have to stick to this no-cheaters rule when some of the most acclaimed love stories on film--and on paper--had infidelity in spades? A couple of whopping examples off the top of my head:
Gone with the Wind: All right, it's more like historical fiction masquerading as a bodice-ripper, but it's still considered to have set many a standard for romance. And, dude! Scarlett marries two other guys before giving Rhett a chance--stringing him along all the while--and, in the meantime, does her best to seduce Ashley, a (mostly) happily married man. This would never fly with a modern romance editor. But it's a great book, and Scarlett's ruthless, clueless flirtations make for a ripping good read.
Sleepless in Seattle: Again, held up as a contemporary classic of the romantic film genre. But Meg Ryan's character, throughout, has a fiance, a nice guy, who she's sleeping with throughout most of the film, and lying to about her crush on this stranger in Seattle. Again, romance editors would send this a tidy rejection letter. But if she didn't have the fiance, she'd have no particular reason to be so conflicted about checking out Tom Hanks, and you'd have no story.
Can you think of other examples? Do you have non-negotiable rules for the love stories you read? Or are there no deal-breakers as long as the story is well written?