the birds and the bees and the flowers and the trees and signs

Yesterday A learned to his shock that not all dogs are boys, and not all cats are girls.

Mom:"Really! There are boy and girl everything!"
A: "Everything?"
Mom: "Everything!"
A: "Boy and girl zebras and fish and elephants?"
Mom: "Yep!"
A: "What about plants?"
Mom: "Uh... yes! plants have boy parts and girl parts."
A: "What about signs?"

The only boy and girl signs I could think of were the ones on bathroom doors. Uh oh.

Mom: "Um, no, signs are not boys or girls. They're just signs."
A: "Ha! Gotcha! you said everything! You were wrong!" This is followed by evil mastermind laughter.

Help me. He's only five.