If our moms are perceptive enough to know when your fly is down she probably notices when you have a boner. amirite?

a boner fly up

This requires either getting a boner while with your mother, or already having a boner and choosing to enter a room your mother is in.

Nooooooooooo!!!!

Uh oh. Your mom must be in trouble. She knows about deez

😆 fair enough!

Only your mom does that, and not even to you, only all of us. Sorry to break it to you this way.

Plot twist- she was the first person to see your boner.

This isn't anything. Imagine falling asleep naked after having jerked off as a 13 year old, and then having your mom walk in with your shmeat hanging out.

What is a fly, why would I have one, and why would it be down?

It depends on how big the penis in question is.