Has sexualizing children ever been a moral act? Has rape ever been a moral act? Has owning people as slaves ever been moral act? The answer to those questions is and has always been a factual no.
Morals vary from place to place and time to time. They are beliefs. “Sexualizing” as used today is a new concept that didn’t exist until fairly recently. The Juliet of Romeo and Juliet was revealed to be 13 years old in the play. If you know of some indication that her age raised moralist’s eyebrows at that time, show us the evidence.
The age of consent is 16 in Washington State (the state just north of where I live) whereas here in Oregon, boffing a 16 year old means prison time.
So, what is underage is a fact in law only legislatively. It is not a fact in the scientific sense like all metals expand when heated.
As for the morality of rape, I suspect early on, societies were amoral in terms of sex beyond “Hands off. She is mine to have sex with.” Morality, even today, functions to someone’s benefit. Did the first male homo sapiens present females with nosegays of daisies or freshly killed rabbits in order to get sex? Somehow, I doubt it. I don’t think something resembling what we today would call rape can be totally set aside.