By that logic, every law leads to moral vagueness. What's "child abuse"? If you spank your child, in some jurisdictions that's fine. In others it's illegal. For all I know, there may be some where it's okay so long as you don't leave bruises. Does that prevent us from making justified accustations of child abuse?
What's spousal rape? In some jurisdictions there is no such crime. In others it is a crime.
What's copyright infringement? Again, the details differ by jurisdiction. Different jurisdictions recognize different fair use or fair dealing considerations, and have different lengths of time for copyright protection.
> calling you a killer, because you kill insects
If I killed someone, and you call me a killer because of that, by pointing to the dictionary and legal definitions, can my friends defend me by saying that the definition is overly vague?
FWIW, I am a killer of insects - Killer is a not-uncommon nickname, https://en.wikipedia.org/wiki/Killer_(nickname) - I am a criminal because I've been ticketed several times for breaking traffic laws, I believe there is a strong argument against defining sexting as the production of child pornography, similar to the argument allowing some minors to have sex with adults of similar age.
I do not believe there is a strong argument in saying that sex with someone who is unable to grant consent is not sexual abuse.