Before they came together she was found to be with child.I often try to imagine this story from Joseph's perspective. He's just been bethrothed. He's dreaming of his future with Mary. He's making plans for his life. And then he finds out that his bride is pregnant and he's not the father. How tough would that be?
As I read, I know how the story turns out even before Joseph does, and Matthew's telling doesn't leave even him in the dark for long, but when I try to imagine the story from Joseph's perspective, I know that life is lived in that long pause between when he find's out Mary is with child and when he meets an angel in his dream. And beyond that, he lives the rest of his life without really seeing the fulfillment of these things.
Now as Matthew tells us the story, he says all these things happened to fulfill Isaiah 7:14. In context, Isaiah 7 is a story about Judah in a time of crisis. Things aren't going the way they'd hoped. Assyria is a looming threat in the world, and Israel is trying to force Judah into war. Into this context, Isaiah tells King Ahaz to stand firm in his faith in God because "God is with us."
If Joseph were anyone else, that could have been all he needed to hear. "Joseph, things aren't going the way you planned, but stand firm in your faith. God is with us."
This is where I see the story's relevance for me. My wife isn't going to bear the Son of God, but there will be times in my life (more of them than I'd like to admit) when things aren't going to go the way I'd planned. What can I do in those times? I can only trust in God, knowing that whatever happens, God is with us.