The French also AGREE with the fact that an adult should not have sex with a child and that's why adults who get caught having sex with children go to JAIL.
The only difference is that the French still differentiate a violent act from a non-violent act, allowing them to punish the actually actions according to their gravity.
Let's take a much less emotional example:
There is a difference between a simple assault and assault with a weapon. Anyone found guilty of a simple assault (punching someone) will spend less time in jail than someone who is guilty of assault with a weapon (shooting someone). Nobody argues the logic of this because it is OBVIOUS that getting shot or punched normally don't cause the same amount of damage to the victim(s).
In the case of physical assaults, everyone agrees that the punishment should match the gravity of the acts.
Another example: We do not treat someone who stole a candy bar the same as someone who robs a bank at gunpoint even if both crimes are, at their core, the same: Stealing something form someone else... We take into account the circumstances of the crime and the acts that took place and the damage it made to the victim(s).
In the case of theft, everyone agrees that the punishment should reflect the gravity of the acts.
As much as we all despise adults who have sex with children, the same logic must also apply to those cases and they ought to be judged on the basis of the precise acts that took place and not imaginary acts that did not take place.
If you treat someone who rapes a person at knifepoint the same as someone who rapes without violence or threats, at one point there is no reasons for rapists to not use violence in their next rape because their outcome will be the same.
Of course it is very difficult to not get angered at adults who target children and to keep an objective mind when faced with the abject crime that it is. But being objective is precisely what justice demands.
If people feel the there is no justice, they stop believing in following the law.