He wants to have a 3some

What ya think? My spouse Wants a 3 some not only that he wants to have another girl next to him. I'm in disbelief that he had the nerves to ask me such thing!! He said he found a girl that would like to include into our relationship. I don't even know what to think of that. I obviously said no! But he keeps insisting. He says it's a good way to keep our relationship going without cheating behind my back.