On October 13th, just in time for Halloween, TeamKRAMA (visit their Patreon here) released a game called Seed of the Dead. (The original Japanese title of the game was going to be 'Rape of the Dead', but the development studio wisely chose to change the title prior to release.) In this game, the player controls a survivor of the zombie apocalypse and awakens to find three girls named Hikari, the red-head, Kirara, the blonde, and Aya, the purple-haired woman. The four of them set out for safety while fighting off waves of zombies. However, Hikari, Kirara, and Aya were bitten by the zombies as well. They are unique in that they didn't turn immediately, and can remain normal, as long as the protagonist has sex with them. Antibodies in them allow them to regenerate during sex.

                                   Hikari                                                                               Aya                                                                                         
 

Kirara

Between, or even during, waves of zombies, the player must have sex with the other survivors to keep them human. It's not just fun, but also important to survive. It is a very well-made hentai zombie FPS game. While the characters are in anime style, during sex, they're rendered in 3D and players are given a POV as the protagonist.

(Gameplay from Southside - the nudity is censored for YouTube unfortunately)