A couple questions regarding pre-18 sex.
1. If you had it to do over, would you have had less of it, more of it, or the same of it?
2. That which you had, do you regret having had?
I'd rather have a school of sexually active teenagers than a school of teenagers trying to deal with hormones while living the life of monks.
ISTM there are four objections that might be made to pre-18 sex, none of which justifies the law imposing age-based line.
Objection A: The religious one about idolatry. (The real reason for the adultery prohibition is that sex with a married person violates a covenant with God, and thus breaks the Great Commandment.) It's a valid argument (at least if you buy a certain kind of Christianity. But to the extent it is valid, it applies to everyone, not just to under 18s.
Objection B: Nonconsensual sex. The "what about rape or exploitation or ..." argument. Well, sure. But that's true whether the person is 14 or 54. Assault is assault. If you want to use "youth" as an factual argument for "unimformed consent" or some such, that's fine. But to say as a matter of law that just because someone is 14 or 16, they're presumptively less able to grant consent, that seems me as wholly unproven.
Objection C: The "kids are too young to bear that quantity of responsibility (e.g. for a kid of their own). Well, okay, I'll buy that. (Though the Admiral Farragut example would be a bit of contrary evidence for that claim.) But, funny thing is, you can have sex without it yielding kids.
And sex education isn't rocket science: Its "if you put this bit here, and you don't put that bit there, this little bit here is going to become a kid." And the economics of childrearing isn't that hard to get across either: here's how much a kid (you) cost me when you arrived (show bills, declining bank balance, etc.).
Objection 😨 STDs. Pretty easy education, too. "This is your penis. This is your penis on gonorrhea."
And do not be conformed to this world, but be transformed by the renewing of your mind, that you may prove what is that good and acceptable and perfect will of God.
Romans 12:2 (NKJV)