It depends on how it's presented. If it's clearly marked as adult - themed fanfiction and not accessible to younger audiences, then it could be argued that within the context of a creative and consensual adult fan community, it might be acceptable. But if it's out there in the open without proper warnings and accessible to anyone, it's unethical. The Naruto franchise has a certain image and these extreme themes can be jarring and not in line with what the franchise stands for in most cases.
Well, in general, any form of fanfiction that involves kidnapping and bondage themes can be a bit dicey in terms of ethics. In the Naruto universe, which is mainly aimed at a younger audience in its original form, such themes are far from the original intention. However, in the realm of fanfiction for an older and more mature audience who understand the fictional and consensual nature of such works in that context, it could be seen as a creative exploration of different scenarios. But it's important to note that it should always stay within the boundaries of legality and respect for the original source material.