I haven't read all the posts, so I'm sure this has been said.
These games do a brilliant job of representing female characters - they can be badasses, heroes, villains, leaders etc. Kind of like IRL. However, like IRL, language is gendered. I think "mankind" is an odd word, but I know that it isn't going to change any time soon, seeing as it's been used to describe all humans regardless of gender for a very long time, and these things don't just change overnight - much like the telling of tales in Skyrim.
At the end of the day, Skyrim is still quite patriarchal (I've had female NPCs say how hard it can be being a woman in Skyrim), so it makes sense that any great prophesies would be assumed to be talking about a man. The fact that you have this alongside great females in history/lore and still living within Skyrim is what makes it a more believable world imo. Much like with races I suppose - I just had a quick look and I don't think there are dragon words for "mer" or "elves". I doubt they expected the Dragonborn to be a Khajiit either!

Sorry to hear that you ended up having insults thrown at you though (welcome to the internet I suppose), you seem polite and although I disagree with your point, I can see your reasoning.