The beauty of having scientifically enhanced characters in sci-fi romance is they bake a “sense of wonder” right into the romantic relationship. It's a clever way to streamline a story's science fictional elements as well as integrate them with the romance.
The technique makes the featured technology very, very personal. Intimate adventures from kissing to intercourse take on new meaning and allows for an exploration of a variety of issues through the lens of a romance. After all, it's not a stretch to imagine the existence of enhanced humans in real life at some future point. Given enough time, funds, and research, genetic engineering may become a part of our daily lives. SFR allows readers to explore such possibilities as well as how they might impact our romantic relationships.
Genetic engineering elements and lab enhanced characters can be adapted for many stories and settings, whether near future or far future, on Earth or a distant planet. It involves the fantasy of gaining superpowers, but ones more subtle than those of Superman. Enhanced abilities include increased strength, bio armor, telekinetic powers, healing powers, or the ability to experience the life aquatic on other planets.
Many SFRs feature heroes and heroines who were enhanced in a laboratory as a result of genetic engineering or other types of scientific experimentation, often of the sinister kind. Lora Leigh’s erotic series The Breeds is one of the earlier ones and it features heroes who were bred, without their consent, with the DNA of predatory animals like lions. You'll find a similar premise in Laurann Dohner's steamy New Species series. And in Sharon Lynn Fisher's The Ophelia Prophecy, the hero is part of a genetically engineered race of humans who share traits with Earth insects.