What's missing from your analysis is consent. As a society, we've agreed that children do not yet understand the complexities of human sexual relationships well enough to give consent and that the massive power imbalance in such a relationship allow informed decisions on the part of a child. And we've agreed upon this because we've seen over and over the damage that's caused by child sexual abuse.

Sex with a child is sexual assault, and we should take it every bit as harshly as we do.