I think that there's a fairly standard distinction, that doesn't require any consideration of psychological mechanisms. Homosexuality can be practised consensually and nobody else needs to care, because nobody is harmed. Paedophilia can't be practised consensually because children are below the age of consent, and can be harmed.

Hence, paedophilia is classified as a mental illness, but homosexuality no longer is (in most Western countries, at least, I believe).