When it doesn't have a happy ending. Full stop. No exceptions. I know plenty of people like and enjoy media that involves kissing but doesn't promise a happy ending. I have certainly been known to enjoy it myself. But just as a mystery would not be a satisfying mystery without ever finding out who did it, a romance isn't a romance if they don't end up together. It can be romantic, heartwarming, and generally enjoyable without being a romance. Much like the folks attempting to hold back the deterioration of the core meaning of literally, people will show up and defend the appropriate use of the word romance, not because we or they are terrible sticklers, but because as a genre definition it has meaning. There are many fans who come here seeking that specific resolution, and they are often willing to put up with the wildest of rides, because the ride has a specific endpoint. It might rip their heart out along the way, but it will deliver them safely to the endpoint.
And let's face it, it's super easy to come up with ways to describe a story that involves kissing or sex or longing looks or whatever combination of the above that doesn't involve the word romance. So, if you use the word romance to describe a story that does not fit the genre definition either you have failed to do your research - which makes me concerned in other ways about your storytelling ability - or you are trying to lure romance fans in because you have heard they are large and mighty and you are intentionally trying to trick them - which also makes me concerned for your storytelling ability if you don't even trust it enough to describe it correctly. 
Rants About New (and old) Things