`When Natsuhi just heard the voice she started doubting caller's gender but after realising who was that person the doubt never crossed her mind again.`
Well maybe she never really knew the gender and since the child called himself her son, she tought it was male. .
And this alone does not confirm Yasu being male, Why do you think Genji hid Yasu from kinzo, He was afraid that he would do the same mistake and surly Genji would not be that crazy to think Kinzo would do the same on a male.
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