My husband said he wants to have a threesome with another woman

Yesterday, my husband talked to me about threesome. He said that he is interested in having a threesome with another woman. I was shocked and so angry. Threesome is three people having sex. He means that he wants to have sex with another woman. I know that there are lots of swinger couples around us. But I never think he is a swinger too. I am so anguished for it. I think he said it wrong. If he says that he wants to watch me with another woman, I think I can accept it more easily.