Would they only deny you a security clearance if your sexual orientation was a secret? It seems like it would be hard to blackmail someone who was publicly gay.
1. One concern could be that even if you are out, you might have partners who are not. The threat of outing those partners might be used to gain some leverage over you.
2. Gay relationships were frowned upon by society. There was none of the societal support that encourages and aides straight people in forming monogamous couples and settle down. The only option for gays who did not want to remain celibate in many cases was to resort to picking up strangers in sex clubs, or dealing with hustlers. That's not the kind of thing that security people like to see, regardless of whether you are straight or gay.
The kids here especially may not appreciate how far gay acceptance has come even in just the past 20 years. The progress from the Stonewall Riots (1969, NY City) to today is pretty staggering.
I hate to break it to you - but the Rooskies already know about space !
The design of the fan blades of the Rolls Royce jet engines on an A380 are a closely guarded secret and yet we allow people to fly on it without a security clearance - even people from Seattle, even Boeing employees (although they do have to sit outside!)
One of the real problems with the Shuttle is/was the cold war paranoia of Nasa. I worked on Hubble and 20years afterwards I still (as a Brit citizen) wasn't allowed inside JPL for an anniversary celebration
Possibly quite a bit. Astronauts aboard the Apollo 13 had to make various repairs to different subsystems, so I would not be surprised if later Astronauts received fairly detailed information about how the entire system worked as part of their training, 'just in case'. At which point, you really don't want to have to worry about which pieces of information are safe and which are classified, or who is allowed to talk to who about what. Much easier and safer to just ensure you can give away as much info as possible to everyone involved.