Just explain to them that the delivery is a personal experience that you want to share with your boyfriend - ONLY! If that doesn't sit well, then try explaining that you will be in an emotional state and will not be able to handle a bunch of people there, even if they are close to you. If neither of those things resonate with them, remind them of the secuity the hospital has. Lol
They might be angry but they will get over it. This is not their moment, its yours! Remember that! If them being there isn't going to help de-stress that moment and keep you calm, then they have no business being there. It's not like they're popping a kid out!