At first, Sebastian may be confused. Demons don't typically deal with pregnancy. However, his loyalty to Ciel would make him take on the role of a protector very seriously. He would be constantly vigilant, making sure Ciel has proper nutrition and rest. He might also start to feel a strange sense of possessiveness over Ciel and the unborn child, which is a new emotion for him as a demon. And he would have to deal with his own internal struggle as his demon nature clashes with this newfound care for Ciel and the pregnancy.
Well, some characters might be amused. For example, Atsushi could be a bit shocked at first, seeing the different interpretations of Dazai in the fanfiction. He might find some of the more out - there scenarios quite strange but also interesting in a way.
Sebastian would likely approach the order with his usual calculating mind. He would analyze what Ciel means by 'love'. He might recall how humans show love in different ways. He could start by being more physically close to Ciel, like standing closer or gently touching Ciel's shoulder. Emotionally, he would try to mimic the emotions he has seen in humans who are in love. But deep down, he is also conflicted as his demon nature doesn't fully understand or embrace this concept of love.
Dazai is not based on a true story. Usually, characters like Dazai are formed through the author's imagination and artistic expression to create an engaging narrative for the readers.