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Can we just admit already that authors writing stuff like guardian-ward turning romantic, teenage brides, too old men lusting after young girls (even if they are portrayed as villains), just don't do it for the sake of historical accuracy ? The author just knew their story wouldn't cut it in a modern setting and found one where it's easier for the reader to turn a blind eye.
1800's London? 1500's Japan? The shipping sheep will gobble it up as long as it's romantic (blergh) enough and it doesn't get too close to their modern sensitivities. Not to mention the (hopefully) small group of people who are actually, with full awareness into that stuff and you can sometimes see them dropping the pretenses entirely.
Let's use Occam's razor for a second. The author isn't writing for accuracy. They wrote that stuff because THEY LIKED IT.
Disclaimer: YES there are stories that portray these issues for the sake of commentary and critique. How to tell them apart from the borderline pedo material? They're not in the YA romance section.