Well, this is pretty lame, honestly.
Steam's got Overgrown Genesis on it already, another zombie eroge with the same post-apocalypse sprite set. There's also Seed of the Dead, an FPS eroge set in a zombie infested city.
If it was denied due to the content, there are plenty of other games already on Steam with even more extreme relationship options. Snow Daze, Onii-chan Asobo, Picture Perfect, A Town Uncovered, SHELTER, anything by Belgerum, and tons more. These are all examples of games that don't even need to be patched to unlock the full game.
I understand Siren not wanting to stress out over trying to get the game onto Steam, but this really shows just how ridiculous Steam's vetting process is.