Of course, as being gay becomes more normalized and less of a secret, FBI wouldn't need to take it into consideration.
How true or not that thinking is, I have no way of assessing, but it does make sense if you consider the premise of blackmail to 3rd parties true. The same could be probably considered for gambling (e.g. "we'll tell your local church group how much you gambled away in your drunken stupor on this weekend").
Why aren't they doing this now if it was really about the potential for blackmail?
I was a carefree recent college grad and sheepishly admitted to smoking weed a couple of times, having been assured by older friends that admitting to some past experimentation was a lot better than hiding it. “Is there anyone you don’t want to find out about this?” was the agent’s response.
“Nah, my parents know…”
Later, a friend who had assiduously avoided even being around weed took ages to get his clearance. We theorized that the holdup was DIA being suspicious that he didn’t admit to such minor drug use, and wondered what else he might be hiding.
Habitual gambling still gets the side-eye for the same reason general financial problems do: higher susceptibility to the temptation of a few thousand dollars to let something slip or to look the other way.
I'm sure there's a lot of moral-highgrounding about everyone outside the group being untrustworthy, and looking for confirmation of this in the most paranoid ways.