All I can say is, "Oops," "They were young and underdeveloped," and "Girls aren't usually orange." I knew from the animal shelter paperwork that I had two girls and three boys. I had assumed that the orange cats were both male because orange cats are more likely to be male than female. Well, this evening I had Mae and Bengal in my lap. Someone hadn't cleaned themselves sufficiently after visiting the litter box. I checked Mae. My husband checked Bengal and said he thought I had been mistaken on my prior appraisal. A quick comparison with Mae and I saw they were the same. Then I checked Ziggy: not the same. Bengal's a girl, Ziggy's a boy.
I'm not too surprised that this happened. I don't have much experience looking at kitty genitalia, and when I was trying to assign gender they were only five-six weeks old. There wasn't much to look at.
My husband and I agreed that "Bengal" is a unisex name, and "Ziggy Stardust" is actually a boy in the song, so that worked out. Now I just have to retrain Ziggy and Bengal to use the boys and girls litter boxes respectively.