You misunderstand when you say "systems to prevent those things aren't necessary." These things are absolutely addressed by anarchists. We're not foolish enough to rely on the goodness of human nature to carry us through. There is no state, there are no hierarchies, but people causing harm absolutely can experience consequences under anarchism. Diffuse sanctions, for example. At worst, they can be removed from the community or group entirely.

But more than anything else, it's important to recognize that mutual aid is just as much a part of human nature as things like rape and murder, and the rate at which rape and murder occur are greatly exacerbated by hierarchies. For example, things like masculinity have to go, and we need to stop putting people in positions of power over others and creating such hierarchies.

And this brings me back to the topic at hand. I cannot conceive of an anarchist who would in any way approve of sexually abusing someone young enough to be considered a minor. Above all, anarchists aim to remove hierarchies, and having a grown adult in a relationship with a young teenager -- this would create such a power differential that I can't imagine any anarchist approving of it or hand-waving it away. The anarchists I know very strongly disapprove of such a thing.

The people we colloquially call "libertarians"(1) on the other hand still seem to support the state as well as hierarchies such as those created under capitalism. In fact, most self-described libertarians I know want to do nothing to address the things you mentioned, as well as nothing to address other harmful things such as the social and systemic discrimination against groups like LGBT+ people, BIPOC, women, and others.

(1)yes, a bit of a misnomer since it would make more sense to call anarchists "libertarians," though no one does, unless we append it with "libertarian left," though even this seems like a silly term for anarchism