Not sure why you are struggling as there is a plethora of great science fiction romance novels out there. You could literally pick any type of scenario involving aliens, galactic warriors or whatever and you'll find an author that probably caters to that niche. Try the Office Aliens series by V. C. Lancaster or the Vorge Crew series by Laurann Dohner. I've heard good things about The Alien series by Tracy Lauren too. Now if you mean like straight up science fiction romance instead of the kinda erotic kind that are everywhere I suppose it does become a little bit more tricky to find the good books, but not impossible. To get you started you can try out some my recommendations:

1. Dock Five series from Linnea Sinclair - Start out with Gabriel's Ghost and work your way through this series if you want a nice blend of romance that doesn't skimp on the science fiction action. The lead is a former captain who gets banished toa prison planet for a crime she didn't commit. She crosses paths with a rogue who is supposed to be dead and the two team up to stop someone who is threatening the lives of everyone by breeding uncontrollable killing machines. The story is different for each novel in the series, but so far all the ones that I have read are good.

2. Wreck of the Nebula Dream from Veronica Scott - Imagine the drama and romance of the Titanic disaster, but set in outer space and you have Wreck of the Nebula Dream. The lead in this one anticipated a nice relaxing holiday about a luxury line, but then the spaceship is wrecked and left stranded in enemy space. The captain and crew ducks out of the situation on lifeboats but there isn't enough for everyone for all the passengers. The lead along with a beautiful woman he meets must then try to escape the ship before it blows up. There are other books in this series too but I haven't read them yet so I can't comment whether or not they are as good as this one.

3. Class 5 Series from Michelle Diener - I'm about halfway through this series by Michelle Diener and so far it is very good. It's not as hot and steamy as some of the other romance stories, but that's ok in my books. I also found the hero to be a bit more likable than the ones in other science fiction romance novels but thats just me. Anyway, the story is about a human woman who is kidnapped from Earth and ends up befriending a kinda homicidal AI that helps her escape and leads her to another group of aliens. I kept things a bit vague as the story is pretty neat in my opinion.

I hope these suggestions get you on the right track but if you want more just leave me a reply and I'll see what else I can dig up from my kindle library that might appeal to you.

A Scientific Romance by Ronald Wright, in which H.G Wells's time machine is real, and it's coming back....
The romance is all in the back story, but it's beautifully written and a great science fiction book that would make a brilliant film
It's out of print but worth searching for second-hand.

You can try After Eden and its second part Chasing Stars by Helen Douglas

If your experiences are anything like mine the problem is not finding science fiction romance books but finding ones that are actually worth reading. My kindle is full of science fiction romances that I've started reading and then just gave up because they are so bad. It's one of those genres where anyone who has ever strung together two sentences think that they are qualified to write a book. No disrespect to the best-selling authors in the catagory, but there are some real stinkers available. But I'm glad I can help someone else find the actual good ones as it took me entirely too long to discover which authors are worthwhile.

Author: T. A. White

Novels: The Firebird Chronicles

Reason: I absolutely adored the Firebird Chronicles, starting with book 1, Rules of Redemption. It's an actual science fiction series, so not just filled with random damsels falling for random biologically compatible aliens. The protagonist here is Kira Forrest, an ex war hero who retires to the simple lifestyle of salvaging alien ships. Her life becomes more complicated when she's forces to make a repair stop at a space station and she becomes entangled with the Tuann. These aliens are not just smarter and more advanced than humans but they also don't trust us in the least and Kira could be the only one keeping the peace treaty with them going.

Author: Amanda Milo

Novel: Contagion

Reason: I like my science fiction and I like a good romance but I also like a good laugh so I really enjoyed Contagion. Imagine an alien who is a complete germaphobe getting abducted and finding himself waking up in an extremely unsterile environment. Luckily for the alien there is a human woman in the predicament with him who is actually quite understanding of his situation. I know that this sounds like the worse ever premise for a romance but just give it a shot and you'll see why I adore this story so much.

Author: Ann Aguirre

Novels: Sirantha Jax series

Reason: The Sirantha Jax series is a bit of a guilty pleasure for me as the stories are not terribly unique but I just love the characters and setting so much that I can't get enough. Sirantha is a woman who has this ability that allows her to jump ships through grimspace but then she ends up in a horrific crash that kills everyone onboard her ship. She wakes up as a prisoner with no memory of the crash and is then freed by a bunch of rogues who want her to help them in their mission to overthrow the established order. From here things kick off with plenty of action as well as some nice steamy romance.