Robin D. Owens, Janet Miller and Kaitlyn O'Connor are some of my favorite authors of SF romance. (If anyone is unfamiliar with the romance genre in general, don't expect them to be tame, these all have sex.)
Aside from the happy ending, here are some other common conventions of the genre:
- There will typically be alternation between the viewpoints of the main female character and the main male character, whether the p.o.v. is first or close third. Or if it's a gay romance, the two main male characters or two main female characters. But the point is that these two characters get the lion's share of the screen time, though there may be a few small sections from the viewpoints of the villain (if there is one), a rival (if there is one), a secondary couple (if there is one), or some sort of best friend/sibling/matchmaker/sidekick character. Male writers also have to be aware that the genre has a primarily straight female readership who want the books to explore female issues of mate selection and male attractiveness. Something that's simply a gender-reversed hero monomyth where a female warrior kicks ass and wins a male maiden may _look_ like a romance novel, but it tends to miss the cultural point of the genre.
- There will typically be a limit on how much violence, horror, tragedy, or other dark stuff is allowed to happen in the story. The backstory can have that stuff, but if the main story has people getting slaughtered, tortured, or raped, it's probably not in the romance genre. That's a really common reason for slushpile manuscripts to get rejected, the author wanted to start with a bang so they start with a rape or murder shown directly to the reader, and this doesn't go over well with romance audiences. Also you don't want to have caused severe trauma to the main characters and stressed out the readers during the middle, this can violate the genre's contract with the reader and also make it difficult to plausibly deliver a truly happy ending. A bit of melodramatic angst is great, character abuse is not.
- Some kinds of sf tropes, like truly alien aliens, political backstabbing, gritty war, or dystopias, don't really have much of a place in a romance story. Which is probably why sf romance tends to come across as pulpy, cheesy, or fluffy to someone who likes hard sci-fi or high fantasy. Romance works really well with comedy, adventure, and mystery; it's halfway compatible with thriller/suspense, and that's what a lot of urban fantasy is; it's not much compatible with serious drama. Yes you can combine a love story with those sort of things, it just won't be genre romance. It really does need to be about a positive theme like love solving problems and people finding ways to be happy and fix their world.