You don’t have to “buy” it. It’s just a fact. It’s also not about the sex itself, it’s the whole nature of a sexual relationship. A person of twelve can get pregnant, that does not mean it’s okay for an adult to have a sexual relationship with that twelve year old. The appear to nature is the weakest argument one can make because it’s not just a “tad different”. There are a lot of things humans did when we still lived in the Stone Age, that doesn’t mean it’s right. We’ve changed this because of our current understanding of humans development, whether you “buy” that or not.