What you are saying is that the government should invade our liberties in order to preempt potential threats. That rationale leads to tyranny, as the government usurps increasing amounts of liberty - and, in the process, consolidates more money and power for itself - under the guise of "security." As Benjamin Franklin said, "Those who sacrifice liberty deserve neither."
The distinction is between non-consensual and consensual sex. If sex is consensual, then there is no harm, and thus no crime. If sex is non-consensual (as it often is involving a child incapable of thinking for himself/herself), then there is harm, and thus a crime. Criminalizing rape is the only just deterrent against pedophilia. Criminalizing pedophilia itself (which is not always a crime, as "minors" are certainly old enough to desire and engage in consensual sex) runs the risk of punishing innocent people.
"Yesterday we obeyed kings and bent our necks before emperors, but today we kneel only to truth." - Kahlil Gibran
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