An underrated but really good science fiction romance is 'The Time Traveler's Wife' by Audrey Niffenegger. The male lead has a genetic disorder that causes him to time - travel involuntarily. His relationship with his wife, who has to deal with his sudden disappearances and reappearances at different ages, is a beautiful exploration of love, time, and fate.
I also highly recommend 'Outlander' by Diana Gabaldon. While it has elements of historical fiction as well, it has a strong science fiction aspect with time - travel. The love story between Claire, a World War II nurse, and Jamie, a Scottish Highlander from the 18th century, is one of the most passionate and complex in literature. Their relationship develops across different time periods and cultures, making it a truly unique science - fiction romance.
Sure. 'Warm Bodies' by Isaac Marion is a great one. It's about a zombie named R who starts to regain his humanity after falling in love with a human girl named Julie. The story is set in a post - apocalyptic world filled with zombies, and the relationship between R and Julie is both strange and heartwarming.