You're not wrong, but it's not the "official" usage. Your question is more or less addressed in the Epicene prefixes section of the "Gender reform in Esperanto" Wikipedia page, though probably not to your satisfaction. The gist of it is, that it's generally accepted, but it's not official. I'd recommend reading the rest of the article for a good understanding of gender in Esperanto, and the criticisms and proposed reforms thereof, if you're into that kind of thing.
I'm only going all technical on you to give a more in-depth answer to your question. You'll probably never be given trouble for using "gepatro" for parent, except perhaps from diehard traditionalists, of which I imagine there are a few.