Do not be afraid to take Mary as your wife - Matthew 1:20
It's hard to imagine Joseph not making some assumptions when his fiancee told him she was pregnant
Joseph onlyknew two things:
What would his family think? Would they buy the story? Would he buy his own story?
God saw his heart and knew that he was the right man for the job
As Joseph slept, God confirmed Mary's story and told Joseph to not be afraid to marry her
God allowed Joseph the privilege of caring for the most important child ever born