Pulling numbers out of my ass, but I'd vager the female:male ratio is like 30/70. Could be wrong. Could be more or less.

But yes, far fewer women play than men.

me by saying "im a girl" or "im not a guy".

This happens quite often tbh.

am i really running into that many girls in dungeon groups now?

Maybe, but could be people faking it, or wrongly assumed to be.

even though i play a girl character, random people refer to me as "he" and thats fine

Same. The major reason for this I guess is that its widely known that men also play the female characters. It's a game after all, and I want to play the races with the most pleasing aesthetics.

should i not be assuming most people are guys anymore?

You should. And if they say they're a girl, you refer to them as "she" or "her" because it doesn't matter in the end. If it's someone in your guild you'll probably know for sure anyway.

Also, the reason many women prefer to stay anonymous is because of the incredible amount of spam they can get if the wrong person figures out they're female.