I believe some men are disguised as FUTA because the[y] cant find a lot of straight women but not 80%.
One thing to bear in mind about the "80%" figure is that in American slang, "80%" just means "a lot of," and often is not intended to be interpreted as literally meaning "eight out of ten." So if an American says "80%," best not to take them literally.
Also, I personally prefer not to assume anything about how or why other people do what they do here.
First of all, anyone who is openly playing as a futanari is very clearly NOT disguised.
And although I generally don't label myself as futanari, I have been very open about being a male in a female avi for virtually the entire three years and a half years I have been playing this game. What's more, I can assure you that my motivation for doing so has nothing to do with not being able to find straight women.
I befriend people here based entirely on how they treat me in the game. I come here to make friends and find companionship (rather than just to find a sex partner), and it often seems to me that players in female avis tend to be more caring, more imaginative, and more fun to be with than players in male avis, regardless of who is sitting behind the keyboard.
Of course, there are plenty of exceptions to that generalization, just as there are plenty of people who don't see things the way I do. Two more reasons why I prefer not to assume I understand anyone else's motivation for doing what they do until I get to know them as individuals.