Dear Miss Snark,
I'm often recognized as Satan's sister, so I feel that a question to my brother's agent isn't too far out of line.
If the description in a query letter reads: It's the story of a selfish, but sympathetic, younger woman married to an older stodgy man who meets a handsome young officer, falls madly in love, and runs away with him--would that be mainstream or romance? Because actually it can describe (albeit badly) anything from Anna Karenina to an episode of Desperate Housewives.
So in categorizing a novel that's mainstream, but whose description shortened to fit a nutshell (no insult to agents intended) sounds like a romance, how does one go about it?
This is neither mainstream nor romance; it's boring.
Tone, language, pace and word choice tell me more about category (and your writing strength) than all those slacker nouns and flabby adjectives.
"Chic Central Park poodle collides with overbearing, snotty and yet oddly enticing puggle on the downtown B train to Barkville" is a romance.
If you can't see why that's a romance, you need to read more, and think about how you talk about your writing.
"Killer Yapp is alone. Frantically searching the six thousand stiletto heels on Central Park West at rush hour, can Killer Yapp find the only two that can save him from evil Dog Catcher of Central Park?"
That's a thriller.
Use the language, tone and pacing to convey what your work is. If you're not setting out to write a romance novel, don't call it that.