A romance novel has romance at the heart of it. Typically this means two people falling in love, and the journey as they fall.
Sure, it might be set in a space opera, like Star Wars. But if you take away the love story portion, and you are left with practically nothing? It's a romance.
It is arguably true that you should try to uphold ideals of some sort. Sex after marriage, if you are writing a Christian Romance. Courting the right way, if it's set in a more austere environment--Elizabethan London, for example.
The core, however, should be romance. That's the story your selling. Now. Consider the following.
Depending on where you plan on publishing (thousands or routes, so I won't get into that now), there could be criterion already in place. At what point the two Main Characters meet, when the first date is, the first kiss. Some publishers get almost micromanagingly specific about this. Consider that before writing.
Many readers enjoy the falling in love part. And they want the real deal. No insta-romance, no love at first sight (common complaints I've heard, but maybe your target audience wants that, do your research). They want a strong female character falling for a strong male character. Two decent human beings that find comfort in each other's arms.
There is also quite the market for LGBT romance novels. If you can pull it off, there's money to be made. Do your research if you aren't LGBT, or you could just get people upset, though. My gayday isn't picking up much from the OP.
Consider what makes a compelling couple. Two people that positively affect each other. They compliment each other, they are good for each other. This makes for some timeless romance classics, if done right.
Consider that you can ignore all the above, if you write it the right way. Take as a plausible premise:
A deranged serial killer finds the love of his life in his psychiatrist. She understands him, how he ticks, how he is put together. And he likes that.
(above could be a Joker and Harlequin love story, and while it's IP infringement, and DC Comics won't take well to that, it could be a compelling story if you get it right.)