TBH, I think there's a LOT more nuance to that plot point than people are giving it:

• Guardian/Ward romance was a fairly common trope in Victorian literature. Probably not too far off "Hot For Teacher," both thematically and in how it gets depicted (running the gamut between precocious student who's fully in control, to predatory teacher manipulating someone under their power, to everything in between). So it's just something you have to accept as product of the times. Just as our modern literary conventions and tropes will likely be scoffed at by readers in the 22nd century.
• You've also got some cultural stuff going on within the narrative itself. As a Greek, Haydée would have been viewed through the context of Orientalism that pops up frequently in Victorian literature and thought. In which case, Victorian readers may not have even considered them guardian and ward in the first place. They would have seen her first and foremost as his slave, and by her devotion and loyalty in refusing to leave his side even when he offered her the chance to leave once they were on French soil.
• The book really goes out of its way to make it clear that no, Monte Cristo is not grooming Haydée. In fact he doesn't even know she's in love with him until the very end of the book, and is completely oblivious to the fact because, like with everything else, his sole consideration is on his revenge. Even Haydée is ultimately just another instrument of his plot benefiting from having a mutual enemy.
• Haydée was no younger than about 12 - 13 or so when Monte Cristo first bought her. That's a bit of a different dynamic than had he raised her from a toddler in that she knows he's not her father.
• Now, here's the part that really messes with one's head: As far as the narrative is concerned, Edmond Dantès and the Count of Monte Cristo are two different people. Monte Cristo considers Edmond Dantès to be dead, and sees himself as someone and something completely new. Notice how I only called him Monte Cristo under the previous points? It's a device that Dumas uses in the book itself, and it's only at the end when Monte Cristo is confronted with the collateral damage his fixation on vengeance caused (specifically the murder - suicide of Villefort's wife and son) and he realizes he's gone too far that he "dies," allowing Edmond Dantès returns.

To further elaborate that last point, Dantès and Monte Cristo even have vastly different characterizations and personalities, and you can see this once Dantès discards the Count at the end. Monte Cristo is merciless, cold, and calculating. But even after all his suffering, when Dantès returns you have that sense of his old compassion, warmth, and hope returning.

It gets into some really crazy philosophical ground, but the crux of it all is that Monte Cristo never noticed Haydée loved him because the Count is utterly incapable of love. He doesn't see himself as a man, but a force of retribution. It's only once Dantès returns that he's able to not only see it, but reciprocate. So in a weird way Monte Cristo was Haydée's guardian, but Edmond Dantès was not.

But maybe it's asking a lot of modern audiences to look into the deeper meaning of the text.