This sounds like an argument for anarchy.

Not really. Anarchy isn't lawlessness; it's the absence of rulers. And saying that there are bad laws isn't saying that law, in itself, is necessarily wrong.

The law is full of prohibitions, as in prohibitions on murder, theft, rape, and (yes) tax evasion.

So, do people not murder anymore?
Do people not steal anymore?
Do people not rape anymore?
Do people not evade taxes anymore?

I have news: People still do all of that. As they do everything that is illegal.

Prohibition doesn't work.

It's something we evaluate on a case-by-case basis. The US tried prohibition with alcohol, and even those who never strayed from their belief that alcohol consumption was evil understood (or many of them did) that the prohibition didn't work for their desired ends -- or had consequences that were worse. So they reversed it. A prohibition on certain hard drugs is a case that's still up in the air. Although there are plenty of injustices in the system, prohibiting murder, theft, and rape on balance are a good thing.

If your goal in prohibiting something is to reduce it to zero (or at least to minimize it) then no, even those prohibitions don't work.

If your goal is to punish the act ex post, then yes, they work. That is what jail time, fines, and other penalties are: punishment.

If, even if your ideal would be to punish the act, the character of the act is such that the costs of enforcement are high enough, then you may pragmatically accept legalization even as you keep a personal "anti" position. This is what happened originally with gays, is a common position now on abortion, and is starting to happen with drugs. Obviously your position is that no cost is high enough if we can jail even one sexual partner of even one child; but it would be a tenable position to be pro legalization because of the enforcement costs.

Mine (and Dissident's, as far as I can tell) own position is that contact isn't something which should be punished at all, of course. So in that sense our opposition to prohibition is in principle, rather than pragmatism. But to believe that this one prohibition lacks the same pragmatic components which every law in the books has, as you do, is more than misguided. It's just a lie. And you're intelligent enough that it isn't simply lack of information there; it's that you truly are willing to sacrifice a significant amount of civil liberties in the altar of child chastity.

I never suggested that we just believe the accused on the subject of whether sexual activity took place -- that requires the usual standard of proof.

The usual standard of proof in the Anglosphere 2018 is "she says so, therefore it's true".

prohibition of adult-child sex means we believe the girl if she says she didn't consent.

Yeah, and the law does not admit that she says otherwise.

And yes, we might still punish the man even if she did consent, though I suggest prosecutorial discretion be used more than it is today.

LOL!

Yeah, well, prosecutors have an incentive to get as many convictions as possible. Not to be lenient, fair or truth seeking. Just to get as many people convicted as possible.