The phrase "sexual orientation" has different meanings to different people, though generally it is used to refer to gender preference (heterosexual, homosexual, bisexual). Because of "sexual orientation" being so closely associated with that and because of long-time bigotry that confounded homosexuals with pedophiles, a lot of people are uncomfortable called pedophilia that.
However, I think I understand the heart of what you asking. No one has a definitive, conclusive answer as to what causes a person to have the mental illness pedophilia. But most medical research to date seems to indicate that is is VERY hard to change and that the causes are innate and/or involuntary. Most pedophiles have factors in there childhood in common: Many had head injures when under age 12, (oddly enough, the the same age group they are attracted to) and many were sexually abused themselves by other pedophiles. Whatever the cause, the brain seems to be "stuck" that way and no treatments yet can make their attractions go away completely; they only help them avoid acting on them. In this way, it is similar to sexual orientation (being ingrained, involuntary and hard to change). For this reason, rather than hate all pedophiles, I feel bad for them as I would for anyone else with an untreatable mental illness. I only hate them when the hurt a child or when they get all self-righteous and start trying to justify their actions as normal/healthy.
The reason pedophilia is considered a mental illness and non-heterosexuality is not, is because of the "holy trinity" that defines a mental illness:
3. Compulsion to harm self or others.
Being homosexual doesn't fit any of those (yes a LGBT person could be distressed about being such, but being homosexual isn't the cause of the distress). A pedophile either has distress or they have the compulsion to harm others (children).