Just what is it you don’t believe? That a 13-year-old boy could be virginal, and experience an unbidden sexual attraction?
Unbidden? So he is the victim of his own thoughts?
Sin always starts in the mind. The “orientation” label is used to excuse it at that level. I think better to use the term lust when dealing with sexual thoughts. Just because someone desires to not have impure thoughts in theory, does not mean that he is guilt-free when he has them.
When christians start parsing words to make sinners feel better they do the gospel a disservice. Repentance requires that you hate your sin, even at the thought level.