Scroll down and Stephenie answer's the question herself
it dosent make sense people r saying he drinks blood so the blood has to go some where if thats the case den y cant the other cullens have a child they all drink blood rose has been dying 4 a child wit emmet but......... nothing
I think the blood was replaced by venom, so venom would have rushed to his nether regions to allow the job to get done. It would really suck to live forever and not be able to get any. LOL As far as the bathroom issue is concerned. I think it has more to do with the fact that his body uses everything in his new diet, so there is no waste which means no need to go to the bathroom. We know that female vampires can have sex with humans too. They just can't get pregnant. But this would require some sort of lubrication besides venom, otherwise they would either change their lovers and/or cause them some extreme pain. So perhaps it works the same for male vampires and they retain their ability to produce sperm and their own natural lubricants. Anyway, that's how I explained it to myself...