while reading breaking dawn, I was hoping that bella getting pregnant was going to be something unique only to The cullens, Like since they drink animal blood there chemisty has changed a bit, like there eyes do, and they just didn't realize the effects animal blood had on them, or that because Edward has spent so much time with a Human, his body changed , the venom was somehow affected by the chemistry they share. but I never once doubted that it wasn't possible, Also vampires have so many special powers, I even thought , I wonder if maybe in edwards transformation the venom made it possible for him to have live sperm, anyways to me there a dozen possible reasons why Bella could got pregnant, so it did not take me out of the book, I was fine with it.
Look at any science magazine, you read about animals all the time changeing, and surprises happening, so even in a scientific way its possible..
ALso I just wanted to point out as a guy, that men get erections because Nitric oxide in the body is released. which then allows blood to flow through the veins, so even though thery're vampires, they must still have certain human functions which work, hence sperm must somehow stay alive, but like any animal species the number one goal is to pass on your genetics, so I guess it holds true for vampires as well.
I will Always be waiting in the wings