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The one mature thing to do is Bro-Zone him:
He will know that you are straight, and still feel like friend of yours.
Personally, o would be Honored to be attracted to any pokefan, most of them are pretty cool Dudes/Dudettes, but since i am straight, i could only feel the same way to a female Pokemon fan.
>TL,DR: Turn him down gently, but feel good about yourself.