Most of the ancient commentators of the Bible interpreted it as meaning that Joseph was law-abiding, and as such decided to divorce Mary in keeping with Mosaic Law when he found her pregnant by another. However, his righteousness was tempered by mercy and he thus kept the affair private.

Did Joseph have a wife before Mary?

The Eastern Orthodox Church, which names Joseph's first wife as Salome, holds that Joseph was a widower and betrothed to Mary, and that references to Jesus' "brothers" were children of Joseph from a previous marriage.

How was Joseph chosen to be Mary's husband?

The betrothal legend in the Golden Legend

The High Priest ordered them to each bring a rod; he that owned the rod which would bear flowers was divinely ordained to become Mary's husband. After the Holy Spirit descended as a dove and caused Joseph's rod to blossom, he and Mary were wed according to Jewish custom.