Are the nativity accounts pious fiction?

Unbelievers regard the nativity accounts as pious fiction. For the sake of argument, let's bracket the supernatural elements and consider the realistic elements that remain:

i) If Mary was pregnant out of wedlock, we'd expect Joseph to divorce her. In that culture, not only would her condition bring shame on herself, but dishonor her fiancé. So that's realistic. 

ii) An angel appears to Joseph in a dream. In that culture, people believed that God sometimes communicated to individuals through dreams. Even if you don't believe in revelatory dreams, there's no reason to think the narrator invented that, since ancient Jews and pagans had dreams which they took to be omens, &c. There's nothing unrealistic about Joseph having dreams which he interprets as divine messages. 

iii) Ancient people believed in portents and prodigies. If, therefore, the magi saw something in the sky which they took to be significant, it's not surprising that they acted on it. That's realistic, given cultural assumptions.

iv) Even if modern unbelievers deny predictive prophecy, ancient Jews did believe in prophecy. That included belief in a promised messiah. So there's nothing unrealistic about Herod consulting priests and scribes, who, in turn, quote an oracle. 

v) Even if Herod didn't believe in prophecy, his subjects believed in prophecy. He was an unpopular ruler. His subjects considered him to be a usurper. So he'd be paranoid about a perceived rival. That's realistic. 

vi) Even if unbelievers don't think Matthew's prooftexts are really about the messiah, it doesn't follow that ancient Jews didn't think his prooftexts were about the messiah. So that's realistic. 

vii) Likewise, assassinating the child at the site where Jews believed the messiah would be born would be a logical way to squelch the rumor. So that's realistic.

viii) Killing all the boys 2 years and under gave him a margin of error to ensure that he eliminated his rival. So that's ruthlessly realistic. 

ix) If Herod was known to be a threat to Jesus, his parents had no choice but to flee for their lives. So that's realistic.

And if they fled the country, to escape Herod's jurisdiction, why not take refuge in Egypt? Indeed, God protected the Patriarchs in Egypt, so there'd be divine precedent. That would make sense to Joseph. 

x) If Joseph thought Herod's son (Archelaus) posed a threat to Jesus, it makes sense that he'd relocate to Galilee rather than Judea. So that's realistic. 

When you think about it, some of Matthew's prooftexts could derive from Joseph's understanding of prophecy. Joseph's actions might be guided by his own understanding of ancient Jewish oracles. 

As a refugee in Egypt, he might well reflect on Bible prophecies about Egypt, especially concerning God's son (Hos 11:1). 

Likewise, suppose that Mt 2:23 is based on folk etymology. You have folk etymologies in the OT. So it's possible that Joseph's choice of where to reside was guided by that conventional hermeneutic. 

xi) If there was a census requiring the tribe of David to rendez-vous at their ancestral hometown, the custom of ancient hospitality would mean lodging with relatives. There were no hotels. But by the same token, homes of relatives would rapidly fill up, so it's realistic that Mary and Joseph had difficulty securing accommodations with their kinfolk. 

Now, if so many elements of the nativity accounts are realistic, then it's unreasonable to assume that Matthew and Luke fabricated this material–even if you don't approach it from Christian presuppositions. That's a gratuitous assumption, because it isn't necessary to explain the material. If an account is realistic, our default premise is that it's realistic because it really happened. Moreover, it's realistic based on what would motivate the participants, and not necessarily what would motivate a modern reader in their position. 

Finally, if the accounts have that much realistic detail, then that lends credence to the supernatural elements.