Full disclosure, futanari is one of my preferred genres, as long as it's done right. I never bring it up here because I don't want to use the blog space as my own personal platform, but I write some smutty fiction, and futanari features prominantly. Saying that, I know that futa can be done wrong, and when it is done wrong, there's no fapping. However, Witch Knight Anna does it right. Made by Circle Sigma, it follows a warrior named Anna who sets out to save her friends. However, a succubus curses her, and she becomes a target of the very people she sought to save. They don't want to kill her, they want to have some fun with her. Check out the PV HERE. According to the devs:

The story of the golden wind, that battles against the snake of despair!
Anna is bewitched with a futanari curse, and is violated by the companions
that she had ventured out to save...!

With the theme of "free expression of eroticism,"
we will use a "witch" and a "demon" to depict situations,
in which the heroine ends up in trouble!

Over 400 visual pages!

Official Site:
http://circlesiguma.web.fc2.com/

Twitter:
https://twitter.com/SigumaAwamori

Unlike other eroge, players will engage in "sex battles". It's not a matter of click to go to the next bit of dialogue or CG, you need one hand free to "sex battle".