Maybe 'My Girlfriend's Wonderful Surprise: A Heartwarming Love Story'. It gives a sense of a sweet and positive love story that can be shared with a wide audience.
We could rewrite it as 'My Girlfriend's Surprise: A Beautiful Love Story'. This title keeps the essence of a love story while being suitable for all audiences.
How about 'My Girlfriend's Big Surprise: A Loving Story'? This way, it focuses on the positive aspect of a love story without any inappropriate connotations.