That's... Genuinely complicated.
Kids aren't asexual, and then BOOM they're sexual the second they hit 18. I was very interested in sex from an age that would make most people deeply uncomfortable to think about. Romeo and Juliet laws exist because we recognize that first, kids are going to be sexual, and second, it's not always going to be with peers that are exactly their own age, and that prosecuting minors for statutory rape--since neither party could legally consent--is a little crazy.
So there needs to be some kind of line between recognizing that kids are sexual, and adults not treating them in a sexual way.
Here's the basic line of thought:
Men occupy a more powerful position in society due to the generally patriarchal structures. Women occupy a less powerful position than men, even when a particular women holds more overt power (e.g., a woman that's a CEO). As a result, sexual relationships between men and women always have a power imbalance; that imbalance of power means that women can never really be consenting, since there's always some form of 'threat' involved. A woman that believes she wants sex believes that way because society has conditioned her to be that way, rather than that being something she chose in a vacuum.
And theoretically, this is all true, kind of. But it also isn't, because that would mean that women can never have any agency over their own body or their own sexual choices. ...Unless they "choose" to be lesbian, which isn't actually a choice at all.