Rosalia can't have children? She no longer has eggs or passes them monthly? How is Bella pregnant from Edward? Does he still produce sperm?

Stephenie Meyer explains this in detail on her website. I'll paraphrase here. Vampires were once human, and therefore are very similar to humans in many ways. Vampires have venom-based fluids in their bodies much like we have fluids in our bodies (tears, blood, lubricants in our cells and other tissue). Therefore, vampires are able to be aroused just as humans are, and male vampires have bodily fluids that carry genetic information and can fertilize a human egg. However, not many vampires have ever mated with a human before, as being that close and intimate with a human usually resulted in death for the human. So whereas female vampires are "frozen" and infertile, male vampires still carry genetic information in their semen, and are capable of producing offspring with a human, if they can mate without killing that human, and if the human can survive the pregnancy. 

See Stephenie Meyer's site for more details. The link is posted below.