You can turn into a futanari for certain scenes but she returns to her base form (dickless female 😔) afterward. As far as not sleeping with men, that’s not intended, but may sort of be possible, but more as a challenge run than anything. The game is really focused on the female-on-male succubus fantasy though, so it’s definitely not the intended way to play. If you like SS but want to just fuck girls all the time, Hex has a “yuri mode” that basically makes lesbian-only playthroughs pretty easy but that game doesn’t have futa content yet.