[libertarians] assume stuff like
What does it matter what a some libertarians assume? In most of my life, I assume that the world is flat. In reality, it's a globe. Locally, it's flat, but I only rarely need to make that distinction. And that I assume the earth is flat doesn't imply that that's something that libertarianism is about. Partly, libertarianism is about letting me believe whatever I want, such as that the world is flat. What really matters, to libertariansim, is the libertarian principle (non-aggression), adherence to the libertarian principle. That it's logically rigourous implies that if one doesn't like a conclusion, one can argue about whether the proof is valid, or about whether the principle is true. Humans make mistakes, they might use false logic to come to a conclusion. They conclusion they might come to may even be true. Of course, one important activity is finding flaws in people's arguments, and a constant re-evaluation of the principle(s).
Now, I'll assume that you meant that libertarianism assumes these things. It doesn't. It only assumes the non-aggression principle. If there is a problem with this principle, I encourage you to let whatever problem there may be about this principle be known.
Perhaps you meant that libertarianims comes to the conclusion that ...
stuff like child porn [is] victimless
Which is a really strong statement. Let's consider whether libertarianism would conclude that child port is victimless. What is a victim? By libertarianism, a victim is one who has been harmed by aggression, or who has been harmed by violence that is in excess of necessity for restitution.
What does it mean for child porn to be victimless? If we mean that the whole industry is victimless, then presently, no. There are victims.
There are, for example, children who did not give their informed consent to be filmed, who, for example, may be psychologically harmed, perhaps only later in life, by the knowledge that someone (probably) has seen them participating in those activities. Or, perhaps, a viewer might stalk them.
There may be children who did not give their informed consent to take part in the sexual activities. In some cases, it seems as though the child is being physically forced or coerced to participate. It may be, as with BDSM porn using adult BDSM porn actors, that the BDSM pornt actors knew what they were getting into, have a safe word, et cetera. Unlike BDSM porn using adult actors, there is no way to verify this. Often, with BDSM porn, there is information about the consent of the actors. (For example, contact details about the custodian.) With child porn, because it's illegal, posting any identifying details about any participants can lead to bad consequences. So we have a situation where there's no way to verify whether all participants geve their informed consent.
There are even videos in which it seems that the child physically harmed.
This is bad, but it is no worse than if the same were to occur with an adult porn actor.