Because you like BDSM.
Would Emiya be considered a pedophile if he seriously went after Rin?
Spoiler:
I was expecting the 'everyone is 18 years old' disclaimer.
But sure, let's play this game. Emiya would be an ebophile, then.
Heroic Spirit Chris Hansen would still totally ruin Emiya.
Spoiler:
Technically I don't think it'd even be ephebophilia, because to have that fetish would mean you're attracted to that age range (15-19) and nothing else, rather than just occasionally falling for jailbait.
Aren't paraphilias fun, kids?
Oh, don't go thinking I haven't seen a lot of weird fetishes.
I think my favourite is that guy who occasionally shows up on /fate/, asking for art of TM girls putting on the mask from the movie The Mask. I remember commenting that it was one of the weirder fetishes I've seen, and someone else got all smug like "that's nothing, I bet you've never even seen 2 girls 1 cup!"
What they didn't get is that the most confusing/hilarious fetishes aren't the "gross" ones, but fetishism of totally innocuous things.
Tickling's something I can get - giving happiness and making someone lose control at the same time. I was amused when I came across a sneezing-fetish fansite for BtVS, personally.
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I once watched a 30 minute fetish video of girls popping balloons with their bodies. I was 13 at the time and could not comprehend how that could be arousing. Some of them were even fully clothed.