Well, A. There's tons of games that don't have any futa or sissification. I'd recommend just blacklisting the relevant tags if it bothers you when you search for content. B. To address the "Oh it's ten percent" thing, the problem is, ANY amount of it counts. For every Seal of Lutellaria or Portals of Pherron where it's the main focus? There's several games where there's one character like that, and that's best case scenario, otherwise it's possible it's only one scene. If you, conversely, are looking for a full experience because everything else doesn't interest you, well the majority of those results aren't going to interest you.
There's a difference between a game focused on futanari/trans like Hardcoded or Futakin Valley and a game mostly just hitting the fetish variety quota. And that's ignoring the headache that's simply that futanari =/= trans in the first place, but apparently the tag overhauling coming ANY DAY NOW ™ will actually separate the two.