Read: Leviticus 20 & Deuteronomy 22
And her husband Joseph, being a just man and unwilling to put her to shame, resolved to divorce her quietly. (Matthew 1:19 ESV)
Little is known about Joseph, but formal education was rare in rural Jewish communities and most boys learned trades early in life. The Bible tells us that Joseph was a carpenter, which basically meant that he built things from wood and stone. Having been identified as a carpenter shows us that he most likely had already passed through the typical age for education and was probably beginning his career. If so, this would place him in his mid to latter teenage years, or possibly his early twenties.
Joseph was betrothed to Mary, so when she showed up pregnant this created a legal and social dilemma for the young carpenter, not to mention the emotional ramifications. Legally Joseph had the right to bring formal accusations against her that would result in her death by stoning. Instead Joseph chose to forgive her and quietly arrange for a divorce. He didn't want to marry her, probably because he was yet to believe her story, but he didn't wish for her to come to harm either.
Joseph was a just man. The man that raised Jesus is shown in this particular example to be a friend to those society is willing to reject and accuse. He had every legal right to punish Mary to the full extent of the law, but instead chose seek a quiet resolution rather than a dramatic and violent conclusion. Jesus is also just. He has every right to throw the entire weight of the Law at us, and instead he offers us his very life. Jesus was raised by a just man to become a just man.