...which is confusing because one would think they can identify genders. The anime has certainly indicated that pokes can become attracted by sight before. (including in this ep) Am I supposed to believe that this is a girly boy or something? Or that all of a sudden they went "oh, there are no gender differences for this one, so they can't tell either, derp" (which would contradict the above) Since this is BW, I am going with the stupidity answer.
