It's weird, but I've heard from a number of FTMs who knew about MTFs but didn't realize that it could go both ways. There just weren't any visible FTMs back then, and for some reason, our brains didn't take the logical leap.
Or maybe we did make the leap but were afraid to think about it.
I knew about Renee Richards, Jan Morris, and Christine Jorgenson by the time I started college, and I never made the connection with myself. I didn't realize what I was until I read a horrible book about it.