Threesome/Swingers

My husband has been bringing up wanting to add someone in our sexual relationship, at first it started off adding another guy because he thinks it's hot for him to be having sex with me and me sucking on another guy, and I told him we could but I was very picky on the guy, had he not brought this fantasy up it wouldn't have never been brought up, but I know how much it means to him, and I'm okay with it. And then it's turned into adding a whole other couple and I keep telling him I could never watch him have sex with another woman. I'm the jealous type, say what you will, but it's just the way I am and I can't help it. He keeps bringing it up and I tell him the same thing over and over. Has anyone ever done this. If so could you tell your experience negative or positive. Thanks.