The reason to “not know” is that you might start preparing for the baby before it is born. This is frowned upon because of “ayin Harah”. We wait until the baby is born healthy before we prepare for it, because we don’t want to give the baby an ayin harah. If you know you are having a boy, you might start painting the room blue, by the crib accessories, by the baby clothes, etc.
Do you think that the same people who can be trusted not to buy a crib and other baby furniture before birth can be trusted not to paint a room blue or buy crib accessories?
I think that if we can trust people not to buy major items before birth (which they’ll need regardless of the gender), then they can likely be trusted not to buy gender-specific stuff.
The Wolf