close the door and never ever mention it again

Leave them be

1)Let them finish, 2)never talk about it and 3)start to talk with them like grown people

I caught my one son doing so at the computer.. turnred away with a "clean up when you're done"

I'd say, oops sorry didn't mean to invade your privacy.

In their bedroom nothing, in the kitchen, please wash your hands

Ummm...Let them finish....then explain to them.

Let them finish, then have a talk.

If they were doing it in a private room, I'd leave them be, but explain porn is not real sex and is not very good sex education.

I would discuss and ask if there were any questions I could answer.