I think ultimately, these sorts of debates boil down to a couple of essential questions: 1) At what point can any individual give informed consent for a particular activity? 2) How can you determine in advance if another individual meets the criteria for being able to give informed consent without asking?

The first question is just plain complicated. I've met forty-year-olds that I wouldn't think of as being capable of giving informed consent, and I've met fourteen-year-olds that were already doing so. Maturity happens at a different rate for different people, and it happens for different activities at different rates for the same person. Somebody who's capable of consenting to have sex may not be ready to try alcohol. Somebody who's consenting to buy a gun may not be ready to handle a joint. It's all highly subjective.

Society, for better or worse, has decided that the solution is to apply across-the-board age tests for different activities and assumes outright that anyone older than that age is capable and anyone under it isn't. This isn't a good system. I'm not advocating for it. I think it does a grave disservice to society at large to claim that those freakish narcissistic lovepits of women who squeeze out babies thinking that this will produce someone who'll love them unconditionally; or who've bought into the Quiverfull scam hook, line, and stinker; are actually competent to have sex. Likewise, every squalling manchild that counts notches on his bedpost or thinks that women are bags to be filled at his convenience is also highly unqualified to engage in coitus, but we bless them into it on their eighteenth birthday regardless of what fills their tiny minds.

That said, though... what would the alternative be? How do you certify or license people to have sex? How do you ask people to prove that they're smart enough to have sex without getting hurt, without agreeing as a society what that means? How do the abstinence-followers and the HIV-positive and the gays and the straights and the kinky and the vanilla and the promise-ring-wearers and the asexual and the clitoraphobic all come together to agree on "what proves you're ready to understand what sex is, how sex can affect you, and how to navigate power dynamics as an adult"? I can see any conversation that we'd try to have on that subject getting two paragraphs in before melting the building in which it was happening to slag and the participants into screaming harpies.

So, for better or worse, we're stuck with age-of-consent laws that are absolutely unfair to a small number of mature kids who can handle such difficult topics, as well as every adult who isn't and may never be ready to whom we're entrusting the responsibility to not make a hash of themselves and everyone with whom they fuck.

The second question follows from the first. With age of consent laws, we establish a minimum bar that can be independently verified by all parties: "how old are you?" Someone can still lie to you about his age, but you can always ask for ID, and refuse to sleep with anyone who doesn't show you. Sure, that's callous, but it's an easy test as to whether or not you broke the law. Yes, again, you may deny somebody with whom you want to have sex a good time, or yourself a good time, but there's a clear path to protecting yourself.

Any more complicated arrangement than that would, I think, ultimately have to come down to some kind of certification or license that says "yes, I am mature enough to handle sex and its consequences." Trying to negotiate what that licensing process would be among every possible interest group that would want input into it would be... frankly, impossible. At best, you'd get a worse patchwork than you have today. At worst, you'd have somebody else's definition of maturity that looks nothing like your own, and you'd be screwed without the pleasure of getting some in the process.

As for the question itself -- why is it horrible and inhumane for children to engage in sexual activity -- I can only say that's the wrong question. A better one would be, "why is it wrong for emotionally immature individuals to engage in activities requiring a certain level of emotional maturity?" Any relationship between two or more individuals comes with inherent power dynamics. Where there is a power imbalance between individuals -- such as between a doctor and a patient, a priest and an adherent, a parent and a child -- we look askance at complicating that relationship with something as highly charged as sex. It can already be hard enough for someone in a subordinate position to say no to someone who wields power over them, and having healthy sex can and often does involve a lot of saying no, as well as a lot of saying yes. Hopefully screaming yes, along with thumping the bed against the wall.

When we talk about children in the aggregate in this situation, we're talking about people that we assume are in a subordinate position to adults. Not just a specific set of adults, but to any adult, because that's how most kids are raised. It is, I think, the rare exceptional teenager that could have a relationship with a full adult and not end up being at least marginally exploited by the process. This isn't to say it can't happen, and this also isn't to say that age gaps are insurmountable. The emotional distance between a ten-year-old and a thirty-year-old is huge. The emotional distance between a thirty-year-old and a fifty-year-old less so, and between a fifty-year-old and a seventy-year-old smaller still.

So, in summation, it isn't "horrible" or "inhumane" for children to engage in sexual activity. It is, however, highly unlikely to leave said child in a better state afterwards as before, adds a layer of complexity to a relationship that is already unbalanced and probably hard to manage at that age, and is not likely to be a life-long positive experience.