No. Creating fanfiction that involves inappropriate or sexualized content about a species, even in a fictional context, is not acceptable. It can be disrespectful to the source material and its fans, and may also violate ethical and moral boundaries.
In my opinion, it's not ethical. Fanfic should be about positive creation, exploring characters in a healthy and respectful way. This kind of description violates the principle of respecting the dignity of all individuals, especially women.
Well, it's hard to say for sure. The concept of 'porn-like' is quite subjective. Some might consider certain novels with excessive sexual descriptions as such, but it depends on individual perspectives and cultural standards.
In many places, 18+ porn comics are considered illegal. This is because they can have a negative impact on society and violate moral and legal standards.