I think 'Ready Player One' has to be mentioned. While it's mainly a science - fiction adventure set in a virtual world, there are romantic elements in the relationship between the main character and his female friend. The book is extremely popular for its amazing world - building and the combination of 80s pop - culture references and a modern - day love story.
A less well - known but still popular among the fans of the genre is 'These Broken Stars'. It's a story about two survivors from a crashed spaceship on an uncharted planet. As they struggle to survive, they also develop a deep romantic connection. The book is praised for its vivid descriptions of the alien world and the slow - burn romance between the two protagonists.
One of the most popular is 'The Host' by Stephenie Meyer. It's about an alien species that takes over human bodies, but within this science - fiction premise, there is a very strong romantic story between the main character and another human. It's a page - turner that has attracted a lot of readers.