It's completely inappropriate. There are many other positive and creative topics to write about, like exploring the beauty of nature, the power of human kindness, or the excitement of scientific discoveries. Writing about sexual content in a 'dirty' way is not only unethical but can also be offensive to others.
It is not appropriate at all. Fanfic should generally be a form of creative writing that adheres to certain norms. 'Dirty sex' content is not suitable for public consumption and goes against the positive and respectful nature that fan communities usually strive for. Moreover, it may involve non - consensual or improper themes which are unacceptable.
No, it is not ethical. Paying for sexual stories, especially those with vulgar content, objectifies and commodifies sexuality in an inappropriate way. It also may promote the spread of unwholesome and immoral ideas.
In modern literature, the presence of dirty sex stories is a controversial topic. On one hand, some may argue that they are inappropriate as they can be seen as vulgar and violate moral and ethical norms. However, others might say that if they are used in a context to explore complex human relationships, sexuality in a psychological or sociological sense, they could have some merit, but they should be presented with great caution and within certain boundaries.