Taken together, the X-Men are an impressive team of talented people. But if you separate them out, it becomes pretty obvious that they're all just a bunch of weirdos who were lucky to find each other. As a group they've done some great things, but take any one individual and you can bet you'll find some pretty embarrassing adamantium skeletons in their closets. We could end this on that super dope Wolverine reference, but then all this mutie muckraking would go to waste.
Though the X-Men have settled into an amorphous mass comprised of freaks from every demographic, at first it was just a squad of teenagers. Jean Grey was only 15 years old when the series started in 1963, but even back then Charles Xavier should have known that developing a crush on his student was mega-creepy. Stan Lee has gone on record saying that he just kinda threw that in there for the hell of it, somehow conflating "the creation of drama" and "skeeving everyone out." Professor X thinks nothing of it in the panels above, implying by omission that pining for a child is not nearly as big a hindrance as leadership(?) or being confined to a wheelchair(??). The plot thread was dropped soon after, and aside from some offhand mentions a few years down the line, it wasn't brought up again until Jean got a peek into her past.
Aided by Onslaught, who's like the Megazord combination of Professor X and Magneto, Jean uncovers her father figure's freaky former fixation. But here's the thing with the "former" part: Professor X never "fell out of love" with Jean -- he just suppressed his memory. Using his powers, he ensured he'd never have to look at himself in the mirror and see a man who wishes he could be a pedophile. If Professor X didn't have the mutant equivalent of that flashy thing from Men in Black, he'd be fighting Cyclops and Wolverine for the favor of a girl he'd been eyeing since she hit puberty.
Fellow X-Men Angel and Husk had a similarly suspect age gap, but their relationship had some more pressing issues...