wolfbiblemoon

That's a good question. At that time it was the custom for Jews to marry within their family line. Mary herself was a descendant of David (http://tinyurl.com/6chj58). As for Joseph he had made up his mind to leave Mary when he found out she was pregnant. After an angel appeared to him in a dream, though, he decided to go through with the marriage and raise the child as his own, as a Jew and with his family name (Matthew 1:19-21.) It was more a matter of legality or public record than of human lineage. I'm sure there is some analogy in there of how God adopts us as children through His son Jesus.