So this is what
really went happened in
Kingdom Hearts: Birth by Sleep:
Old grandpa Master Xehanort begins to get all pedophillic (or more like
ephebophilic to be technically correct, since pedophilia is sexual attraction for children ages 13 and under, while ephebophilia is sexual attraction for teenagers ages 14-19) for his students, and starts tapping Ven's and Terra's assess. He makes Ventus his bitch for a while until he finally gets what he wants from him—Ventus's pure darkness—and then dumps Ventus for his scary, pure darkness form called Vanitas. So old gramps here uses Vanitas as a creepy sex toy for a while. He even uses Vanitas on Ventus.
Then gramps proceeds to molest his other student, Terra. He sees that Terra has a cuter shota baby face than Ventus does. Terra's body was also firm, sexy, and well built, kinda like the artist Caravaggio's boy art models/apprentices.
mmmmmmmm
Terra's cute baby face, along with his body, makes gramps all hard and horny. Gramps also envies Terra's hot, young adult body and wants it for himself. When he finds out that Terra is all emo and moping around because of some problems he's having with the darkness, Gramps saw this as an opportune moment to use his ephebophile powers on Terra. So Gramps is all like, "Why hello there, Terraaaaa...I see you're having problems dealing with your darkness. C'mere, boy, let me help you with your problem, and I'll make you feel all better..."
So then Gramps proceeds to butt-rape Terra. Terra, exhausted from the butt-raping, succumbs to darkness, and Gramps becomes victorious in overtaking Terra's hot, sweaty, flushed body. So now gramps becomes hot, young Terra!nort.
Gramps, now as Terra!nort, then proceeds to hunting down Aqua, so that he can finally feel her boobs without Aqua freaking out that an old man is the one that's groping them. But Aqua still gets pissed off and proceeds to dominate Terra!nort. However, she fails, leading Terra!nort to succeed in topping her and sending her to an orgasmic Realm of Darkness, where she lies and indulges in endless, sensuous ecstasy for nearly an eternity. Terra!nort soon also comes, and he joins along with her in the Realm of Darkness. However, Aqua, humiliated at the thought that her dominatrix powers have been taken away, proceeds to banish Terra!nort back into the Realm of Light so that he can leave Aqua alone to mope in peace.
This causes Terra!nort to get all pissy. He storms through Tifa's Seventh Heaven bar in Radiant Garden, and asks the bar girl for a crap load amounts of whiskey to drink his sorrows away. This gets Terra!nort drunk—and I mean, really drunk—that he somehow ends up on the shores of Radiant Garden, all fuzzy and woozy from the drunkenness. He begins to get all amnesiac and can't remember anything prior to his visit to Tifa's bar. Wise Ansem finds Terra!nort all sprawled out on the shore and proceeds to take him in as his Playboy playmate—err, I mean—apprentice. At this time, one of Wise Ansem's Playboy playmates—err, apprentices—Braig, starts to get all jealous that Ansem was favoring Terra!nort over him. Braig also knew what really went on though. Back then, waaay back then, he made a deal with ol' grampy Master Xehanort on something: harass Terra, and I'll give you the super Keyblade-wielding powers of the POWAH RANGERZ. So this led Braig on having to play as sexual predator on Terra, pissing Terra off and in turn, causing Terra to blast away one of Braig's eye and give him face scars. This pissed Braig off big time, so one day he'll have his revenge on Terra by teaming up with ol' grampy Masta Xehanort and helping grampa Xehy furthur his plan on world ephebo domination. This way, gramps can have all the ephebos of the world to himself, and Braig can proceed to butt-rape Terra so hard for taking away his eye and giving him face scars.
So yeah, Braig proceeds to mess up Wise Ansem's Playboy Mansion—uhh, I mean, castle—by persuading Terra!nort to crash Ansem's upcoming orgy—err, experiment—with darkness, while cautiously, faintly reminding Terra!nort of the secret plan they hatched together on world ephebo domination. So the next night, Terra!nort crashes Wise Ansem's orgy—I mean—experiment, causing all six of Ansem's Playboy playmates—uhh, apprentices—to fall into darkness. This in turn caused the fall of Radiant Garden, and the catapulting of the events of Kingdom Hearts I.
So yeah, fast forward to KH1, Terra!nort gets all weird and even freakier than before, and initiates to prey on little Sora and Riku. He fails with Sora, but succeeds with Riku, and proceeds on butt-raping Riku all the way to Oblivion. But unlike Terra, Riku actually succeeds on escaping Terra!nort's ephebophilic grasp. This kinda pisses off Terra!nort, but he gives up and just goes back to managing his newly set up Orgy XIII. Here, presents himself under the guise of Xemnas, because he likes how it can re-arranged to spell "MAN SEX". Here, within the Orgy XIII, he tells his bitches his plans of world ephebo domination: that once they all gather enough hearts to fill Kingdom Hearts, it will grant them their wish of having loads and loads of ephebos to themselves. This makes all the Orgy XIII members happy, because that meant everyone didn't have to rely on sharing Roxas and Namine anymore. Also, they especially no longer have to rely on sharing and using Vexen's freaky, defective sex dolls (aka Repliku and Xion).
So there you go. The reason why Xehanort is a total ephebophile, especially after knowing the plot of Birth by Sleep.