Why is fiction based on the idea that aliens have visited our planet labeled science fiction or even fantasy while other equally implausible scenarios are not? For example, it is a huge stretch, to say the least, to believe that Jesus of Nazareth had children who are now protected by a secret society. On the other hand, lots of very credible people believe they have seen UFOs. Take today's article in MSNBC , which discusses an international panel of pilots and governmental officials who have called on the U.S. government to re-open UFO investigations, citing safety and security concerns.
I think editors, agents, and ultimately readers should be able to distinguish between something that is clearly science fiction (i.e. a Men in Black scenario) and a situation that may be more plausible than many people want to believe.