Imagine having a modern moral compass and sense of ethics. "So what they did it in the middle ages it's fine in the fantasy story", just because it happened or is realistic doesn't mean it shouldn't make you uncomfortable. And in this case what does it add to the story? They aren't trying to portray a historically accurate setting the female MC has a glowing horn. They could have easily made her 25 or 30, or aged him down to 25 and made her 19 or 20 and it wouldn't have changed the story. The only thing it adds to the story is the ability for pervs to get off to it.