Plot hole: How is it possible that Diana is unaware of the concept of marriage? She has a grasp of multiple languages and cultures and can recite Socrates. Socrates often spoke of marriage, so even if her people do not marry, it makes no sense that she is completely unfamiliar with this human custom.
Suggested correction: She has never seen a man either. She read about them, but never actually saw one. One might question why she doesn't understand a man if she read about them, and yet she doesn't. Same thing with marriage.
She had never seen a man, but she knew that they existed. She speaks multiple languages, despite never having met someone from any of those cultures. My point is, if she's so well versed in world cultures, how has she never heard of the concept of marriage?
Continuity mistake: When the picture of Diana and Steve and co. is taken in Veld, she is wearing her black cloak, but she left that and her other clothes in the trenches, on the other side of no man's land.
Suggested correction: She's in Flanders, in November, and the battle was over. It's cold, and probably worth going back to the first trench to get her cloak again, which they now have plenty of time for.
Except I seriously doubt it would still be here as poor, displaced people would likely have taken it to keep warm, like the woman and child she spoke with.
Continuity mistake: When Diana and Steve are held at gunpoint in the alley, the gun's hammer is drawn back and in the next shot it is in the resting position. (00:53:15)
Continuity mistake: When Steve Trevor is at the Ottoman empire, he is beside a biplane that look like a British one. In the background to his right are a series of Fokker EIIIs, the next shot he's turning to his right, just besides a Fokker... and the British biplane has disappeared. (00:24:23)