Not only that. The Story of the Game has to be written new. In the first 5 min, you get telled that all woman that get raped by/ have sex with Monsters, are turned into Monsters and that Man, with exception of you, turned into breeding/feeding stock for the Monsters. That means with exception of you and the 5 Girls in the starting Zone there are no other "Humans". And if you could play as a Woman, you could not have sex because you would turn into a Monster. With Futa the same. Men can't turned into Monsters in that world, so their are no male Monsters to begin with.1. it's the dev's personal preference, and you can't really do much about that.
2. trying to do so now would likely just hurt the game not help it. Dev would have to go back and add new monsters, debug, add new scenes, debug more, and debug even more (adding gender options would likely mess with a lot of things), which would probably halt actual story progress for at least a year.
3. a wider audience isn't necessarily a good thing. attempting to appeal to everyone typically leads to no one being happy, especially the Dev, because there is no real depth to it. doing one thing well beats trying to do 20 and flopping on all of them.
Sure the Futanari Card for Monsters isn't from the Table for now, but in the End it's the devs decision, what will come and what will not.The Game has, if i'm right, two more Dungoens that are hinted, but not in the Game yet and the Main Base with the Demon Queen. So there is more than enough space for the future. So let ous wait and see what will come.