threesome

ive been in a relationship with my boyfriend for almost 3 years now, our sex life is great but ive always had the fantasy of being with a girl. he is completely not open to the idea of a threesome with another female, and im not sure why. he claims its because he has no desire to be with any other girl that isnt me but i get the feeling that maybe he'd just feel too awkward and uncomofortable? what do you ladies think??