in the bible - carrying on the human race?

Cain ran off and married a woman in the land of Nod, or at least that's where he was headed when he married her. If you are going to go the route you don't even have to go that far, Adam was made by God from dust, Eve was made from Adams rib, thus she is genetically part of him in the closest sense of the word. They had and had 3 sons, Cain, Able, and Seth. Therefore they were uous long before their sons went and had children. This isn't the only in the bible either, I can't recall the names, but I believe it is in Geneses where a mans daughters were worried about their family name not being carried on so they got their father drunk and slept with him in hopes that one of them would become pregnant and bear his children to continue on the family blood line.