Xenogenesis

In the aftermath of Earth's final war, Lilith awakens to see a shadowy figure -- Jdahya, one of the Oankali, a race that has decided to save the human race from itself. Lilith learns that the Oankali are gene traders who interbreed with failing species so that both races can survive. Jdahya proposes that Lilith mate with a third sex in their culture, creating human-alien hybrids who do not have humanity's weaknesses. As her new world beckons, Lilith must decide if survival is worth the price.