Why were homosexuals seen as a national security threat in the united states during the cold war?

Closeted homosexuals have always been subject to blackmail.

Because they could be blackmailed since homosexuals weren't out. Now it's no big deal.

Probably because there was so much hate and bigotry towards them, they were treated as no different than communists at the time.

Because have you ever read about Sodom and Gomorrah? 

Pure prejudice. 

Homosexuality was illegal and you would be thrown in jail if discovered.

So much for the "Good old day's" Conservatives like to reminisce about. 

They were viewed as a threat and a probable inroad for communist ideas. 

My perspective of what you ask, is then they were viewed as not trustworthy due to their unorthodox nature

The only time I remember anything like that was a concern was when someone working for law enforcement or some other branch of government was found to be gay, they were thought to be easily blackmailed. Mostly the society I grew up with were just prejudiced, hateful, always ready to tease, and often ready to hit anyone who they thought were homosexual (they acted against gays like the Left is acting towards conservatives now). You were a pariah in society as a homosexual. Sometimes they would hunt  homosexuals down.  It was a part of the thinking in society that did better once it changed. 

That was back when the Democratic Party was good. Now it's evil. They are not the same anymore.

You're right about the blackmail rational. 

 There was also a strong undercurrent of misunderstanding about homosexuality - it was classified as a psychiatric disorder until the 1960's and perhaps they were also seen as unreliable.