Interest in a ?

So my boyfriend and I have been together for 5 and a half years and I've always had this fantasy of having a with us and another girl. I'm all for it and he says he's all for it but he is afraid that I would be hurt emotionally afterwards but I'm telling him I won't be, I am more open minded than he is though. What do you guys think about this? I don't feel that I would be hurt emotionally but just wanted some opinions on this.