NOTE: I am not saying any of the following are good reasons. Just relaying reasons I've read about before as to why being gay was such a red flag, even if you were out.
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 completely agree that the progress over the last few decades has accelerated dramatically. Unfortunately the number of suicides we still see in the GLBT community, especially among teens, shows that we still have a long way to go. --Mind you I'm primarily talking about the US, here... The situation in many middle eastern and African countries shows just how far the world as a whole still needs to come.
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.