~Edit-
I had another thought. People tend to use confirmation bias as well...
Let's say that there was a boy. He once played with a Barbie doll.
In scenario A, he grew up and was straight. The Barbie doll is forgotten.
In scenario B, he grew up and was gay. The Barbie doll is used as "evidence".
Not just with other people... those of us that find ourselves to be non-hetero can often find apparent evidence in our pasts to match our identity if we look.