According to an excerpt of Rey Mysterio's book published by the UK Sun, Tommy Dreamer thought that Rey was a butch, flatchested young lady when they first met.
A young lady who he was very attracted to.
The version I heard runs like this. Early on I was riding in an elevator with Paul E. I hadn't been introduced to most of the guys yet. Tommy Dreamer saw us get out, gave Paul E a puzzled look, then turned away as we walked on. Later on he went over to Paul E and asked: "Hey, who was that chick you were with? She looked hot. A little flat-chested, but hey." He lost interest real fast when he found it was me. It's a good thing he told Paul E. The boys would have set us up for a date, then hidden around the corner to see what happened. That's the boys.
I could understand mistaking Juventud Guerrera for an attractive young lady. I wouldn't myself, but I can understand it. Not Rey Mysterio, though. He has the whole gay twink thing going.