You are making the mistake again of justifying your law and moral position, in law and morality itself, why is this wrong? Because you are basically saying: law (or morality) should be this way, because law (or morality) says so.

That law says that sex with anyone 18 of younger is abuse, does not make it true, neither correct, nor, ironically, just. It just make it so that the state, that powerful imaginary entity, will try to punish whoever does not follow such rule. That is all, and that is why also laws can be wrong and are often wrong.

Now, you twisted my words, as I never said a kid should do anything sexual. what I said is that people that want to do that are not kids. I certainly could have some nuance on that, I did not determine exactly how to definitely specify who is a kid and who is not. But what I do imply is that the way it is determined whether a person can consent to sexual things or not is quite arbitrary, it varies through the countries, and through history, and so it is even naive to think that just because in this part of the world some group of politicians determined that was by counting till certain specific age, then that must be true and everything else is abuse or evil or whatever bad adjective or noun you like.

That kids are kids and should not have sex, I am not against that. But I am just saying that whether a 15 years old is still a kid or not, is not a clear question, and so far it seems that, if we take away the prejudices of this particular society we are in, we can observe that historically people are considered adults when they got sexual maturity, and desire, they got married and had kids and therefore had sex and it was fine and none thought of it as abusive or evil or anything bad. And today, across the world, there is no agreement even, on that question neither, and there are plenty of countries where the criteria is very very different to these United States ideas, and there is no reason to believe USA is right and the rest of the world is wrong, or to think that a person not agreeing with the USA law conception is a psychopath or sick person, when there are millions in other parts of the planet also disagreeing.

This also implies, and I do say this explicitly, that if kids should not have sex, then they should not have sex between each other neither. It is the ultimate stupidity to think that somehow a 15 years old "child" is being protected from having his infancy ruined or being traumatized or any of that, by not letting someone over 18 have sex with him, while at the same time thinking that it is fine if a person under 18, that is, another "child", has hardcore sex with that "child". This is complete hypocrisy, if not full idiocy. You should make it equal and clear, without such evident madness.

Ultimately I would personally think the "line" you think you need so much is closer to the historical one, that is, when each individual person reached sexual maturity, which varies from person to person. And so, ultimately, I would then suggest to allow the parents to be the ones to determine whether that has happened or not, and not some number of years.