Threesome

So last night, my boyfriend brought up the idea of a threesome saying that "it's every guys fantasy" and that "every guy wants more than one girl" told him that it would never happen with me cause I'm not interesting nor do I believe in them. I told him that in my eyes I see it as disloyalty to me  because you are wishing someone else and literally taking action even if I were to be there. When he kept pushing to try, I felt bad about myself, I felt like I wasn't enough for him, as if the opportunity struck and a women asked him to have sex with her he would do it and not think twice. What are your opinions?