I think I've pointed out quite clearly that there are different levels of culpability
WP, ranging from doing something intentionally (for example, murder) to acting with negligence (Involuntary manslaughter). It seems that at least in parts of the US, there is no need to determine any culpability at all for "statuary rape". Why this offence is so exceptional is very peculiar.
No, that's not how criminal law works. If criminal law were to follow this kind of reasoning, then anyone who causes harm to someone would be convicted of crimes even if the court was to find that it was done completely by accident and that they were really at no fault. The driver(s) in every single car crash that caused harm would end up in court every single time, no matter how safe and secure everyone's driving was.
After reading about it, the reason why the crime of having sex with someone that's underage is such a noteworthy exception to the requirement of culpability in some legal systems is apparently because this defense is hard to disprove in court. Absolutely preventing people from using that defense is just a way to secure convictions. However it's effect is to make having sex with anyone who could be underage potentially subject to legal penalties even when there was no reason they should even have suspected that they were underage, which is where the big problem lies.
Again if were we to follow your kind of reasoning then driving a car would be legal, but if you were to cause an accident then you are guilty of a crime no matter how safe and secure your driving was or whether your actions could've prevented it. The only way to prevent yourself from potentially facing criminal charges would be to not drive cars at all, even-though you are notionally allowed to do so. ******* ridiculous.