If you mean why does he sleep with her, the answer is because he didn't know she could get pregnant. The incidences of vampires impregnating humans are rare, and kept mostly in secret; so Edward has no idea that pregnancy is an issue for the two of them. He and Bella consumate their marriage because that's what married people do. Later, he feels guilty for accidentally hurting her, and it takes him a while to feel comfortable sleeping with her again. But throughout the whole thing, he never imagines that pregnancy is an option. When Bella figures out that her nausea and strange dreams might be the result of pregnancy, they are both shocked; no one had considered the possibility. Without having prior knowledge of the possibility of pregnancy, neither of them would have considered the danger to Bella.
Well I dont think that Edward was aware that Bella could get pregant. Just like Rosalie couldnt get pregant because she was practicly non human, Edward problary that he couldnt have kids either.
But in a way it was still a big risk he toke. I mean he couldve got real into the whole little "practing" thing and bite her or something.
Well Edward did not know that if he had sex with her she would 'Get Pregnant and Die' (Mean Girls reference.) ...okay anyway so Edward did not think that she would get pregnant since he was already dead but she was still producing eggs and having her monthly cycle and once they had sex, she found out she was pregnant and Edward called Carlisle to find out what was wrong.
Edward believed that he, as well as all other vampires, were infertile. This would probably be to the fact that vampires are never born but made. Thinking logically it's impossible for a female vampire to become pregnant since they no longer have a heart beat thus a monthly cycle. Also male vampire wouldn't be able to impregnate either since he does not have heart beat either. So Edward thinks it's safe. It was mentioned in Eclipse when Rosalie was having a talk with Bella. She didn't like Bella for the exact reason that she w as giving up the possibility to have children because they all think vampires are sterile. Even Carlisle was baffled and Alice Skeptical.
Well lets just put it like this:
He could have the choice of saving her and he already made a promise of turning her into a vampire.
THTs all i can say