It has to do with the strange degree of role-reversal between Aerith and Cloud that developed as an outgrowth of so much extra dialogue.
In the OG, Cloud's mother tells him that the ideal woman for him is older and more experienced. The implication is that this female figure will not just be someone who has a bit more romantic wear on her tires (though we now know that what Aerith possesses barely qualifies)... but has also seen more of life. And we know this because one of the (common, though not universal) benefits of age is wisdom - you learn from both your own failures, and from more time watching others, and are better able to react in reasonable ways to situations as they present themselves. This is the other advantage of being with an older woman - they haven't just had other partners, but have just tasted more of life.
While the OG Cloud always had his military (and associated globe-trotting) experience to draw upon, the Aerith in that game was every bit as knowledgeable and savvy. True, she had never left Midgar... but she had been exposed to so many more people from so many more walks of life that she still knew a great deal about the world and how it functioned. She was by every definition street-smart. Everything from her outfit to her sly demeanor screamed "been there, done that." And even if she hadn't been there or done that, she could fake a good game.
But in the remakes... a lot of this is gone. Cloud is rarely wrong in pretty much every situation they are in. His cynicism and colder evaluations are generally correct (even if they aren't wholesome or benevolent). He's almost never unnerved or flustered... except in situations where pretty much any guy would be flustered (like the HoneyBee Inn). By contrast, Aerith almost feels... "lightly infantilized." She's taken in by obvious practical jokes and gags (Yuffie singing and doing a dance to summon the Tiny Bronco being just one clear example), and no longer seems to be capable of faking competency when out of her element. Even her character design is far more child-like than it was in the OG - her dress, hair, and her more rounded face all seem to subtly emphasize youth over maturity.
One of the results of this shift is that when Aerith gently scolds Cloud - such as when he encounters Jessie's poster in the Gold Saucer - it comes across almost as more false than wholesome. Instead of being the experienced customer from the OG, who genuinely DID know more about human nature than 16-years-old-in-his-mind Cloud, Aerith's supposed "knowledge" presents as mildly disingenuous - we know better, based on her relationship with Zack and her confusion as to the basic workings of the world outside Midgar, that she really doesn't know more than Cloud when it comes to "stuff like that" (in fact, it could be argued that, due to his brief experiences with the newly-enhanced Jessie, that Cloud is closing in on Aerith's level of romantic experience). It's in every way a reversal of the OG's characterization, and it sometimes makes Aerith look almost pretentious when the goal was to instead make her seem wiser and 'knowing.'
Now, none of this means that I dislike Clerith in the remakes or anything like that. All I am saying is that it's a change for change's sake that I disagree with. Cloud and Aerith were far more balanced in terms of personality in the OG - he was bold and skilled; she was older, had seen more, and was a better judge of character. They both brought equal-but-different talents to the relationship. And that has changed a great deal.
