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.