...is that when period films or TV shows are made of events taking place during these previous time periods, in most cases cultural mores in relationships are usually depicted as being similar to what they are today. And this regardless of whether or not the characters in said films or shows are historically real or not (this is why, for instance, Pocahontas is typically aged at least 12 years in her film appearances).

Part of the reason is to maintain the fiction that our more recent cultural mores are not recent but eternal and timeless, as if somehow built into the fabric of the universe rather than temporary man-made constructs dependent upon the conditions of a specific period of time.

Another reason is so that the protagonists of these stories taking place in earlier eras can be sympathetic, likeable, and more identifiable with viewers of today. It would be much more difficult, if not near-impossible, for a modern viewing audience to look positively upon, for instance, a 55-year-old protagonist who takes a 15-year-old bride or love interest, no matter how good of a person he is depicted as otherwise. Instead, he would be perceived as a bad guy or at the very least, seriously disturbed or "weird." Only negative characters or outright villains are allowed to be depicted as having such relationships, thus maintaining the implication that these are feelings or (in historical context) actions that only only individuals with serious character flaws will have or do.

In other historical cases, with real life personages well known to have much younger paramours, such as Edgar Allen Poe or Charlie Chaplain, their romantic relationships/marriages will be greatly downplayed in the filmed narrative. The only exception being if the screenwriter and/or director has a bone to pick with said personage, and wants to depict them negatively or as "creepy." In such cases, they will go out of their way to depict their age disparate relationships in accordance with the contemporary "victim" narrative.