My boyfriend wants to have a threesome, we've been dating for a year and he's the only person I've had sex with. I'm just not comfortable with the idea of physically watching another girl be with him and sharing him. But he keeps bringing it up and I don't want to seem square by telling him straight up no. I said maybe someday since I'm new to the whole sex thing. I've only had a sex life for a year and one partner. How should I approach this situation?