There's a piece of advice given to writers: Never use a word in your writing that you wouldn't say out loud. It's good advice. Don't use fancy vocabulary words just to sound smart if that's not how you talk. Don't use slang you're not familiar with. I think we can take it a step further...
If you were in a real relationship with a guy who had a prosthetic arm, would you ever, in a million years, call his other hand his "flesh hand"?
Be honest with yourself.
Because I'm pretty fucking sure he'd break up with you immediately if you ever said the phrase out loud around him.
It's gross, it's weird, and it's fetishy in a really bad way.
A human being with a limb difference does not need to have language like this used about him, so don't do it to a fictional character. You wouldn't do it in real life, so don't do it in your fanfiction. Just call it his right hand. It's that easy.
