Threesomes...?

I'm 22 and have been with my boyfriend for almost 2 years. I dated women exclusively before I met him. I love him so much and can see definitely having a future with him but I find myself thinking about women at least (the very least) once a day. Not always about having sex with women but just about being with them generally. I've considered having a threesome just to 'satisfy my appetite' (for lack of a better phrase) but he's not into it at all. He thinks I'll like her more and it will mess up what we have. I tell him he's wrong but.. he might be right. Any advice? Stories to share? Help!