I have this feeling that.......... I am the one who stumbled on that campfire.
"Oh... Girls... I am so sorry I've interrupted you," I would say. "Really... you don't have to quit on my account. We could just pretend that I am not even here. Really. I'll just stay really quiet over here. Unless, of course, either of you needed any assistance in any way. Then you would only need to ask and I would do what ever I could to help you attain what ever orgasmic plain you were trying to attain."