Why do some people get upset if you accidentally use the wrong pronoun to identify their child?

I was walking down the alley. There was an old, wooden-stepped fire escape that ran up the side of one of the old buildings we have here. It had a heavy iron gate at the bottom of it that was big enough for kids to squeeze through or go under or climb over.

There was a guy, I’m guessing it was Grampa, who was watching a a little kid, about 3, playing around the iron gate. The child had gotten on the other side of the gate, on the steps, and was coming back through to our side, by sliding under the gate.
The kid was dressed in some tan shorts, a plain grey t-shirt, white sneakers. The child had long curly hair half way down the child’s back, pulled into a pony tail. The child had a high, chirpy voice like all toddlers.
I was enjoying the few seconds of action that I saw as I walked past. Just as the child slid under the gate I said, “Look out! She’s escaping!!”
The guy turned and scowled and said, “It’s a boy.”
The look the guy gave me was pretty withering, and I said, “I’m sorry.”

So if it’s important to people that others immediately recognize the gender of their child, and they feel angry when someone mis-identifies them, why don’t they do a little something so people won’t make that mistake?