Vampires can't reproduce with each other because a female vampire's body can't change to bear children. But males have a seman like form of venom that can get a human pregnant.
Question Asker: Where in the series does it say that ? If vampire's are unable to change then eventually male vampire sperm would all die, and this venom bullshit doesn't sound too legit.
It isnt explained in the series, Stephenie Meyer said it in an interview. To read a more detailed explination go to the following web page. http://twilightsaga.wikia.com/wiki/Vampires#Physiology ~Kmanwing
Don't ask too many questions like this...you might be disappointed at the lack of answers.~AndroidBoy420