The Zombie Girl Game Over is an ending which occurs when the player loses to regular enemies during the Zombie Apocalypse Camping Trip.
The battle has left Eric(a) completely exhausted. Yoshida tries to get them up, but is completely overrun by the zombies in a moment, and Eric(a) could only fail to muster the last of their magic before suffering the same fate. Instead of being bitten however, the undead grope Eric(a)'s body, kiss them, and create a new pussy between their legs. The longer they spend in the orgy the colder their body feels, but their lustful heat only becomes stronger. Even when their body function stops, the desire for that warmth is so strong that they jump back into the orgy again, even eating out the zombified Yoshida the moment they make eye contact.
When the orgy finally ends, the now-zombified Eric(a) shambles forward to find that body warmth again to consume and convert more people into zombies. With little connection they have left with Yoshida, the two sticking together as they seek out the few remaining human survivors.
A slightly different ending with different dialogues will play out if the player loses against a Necromancer. She will command the now-zombified Yoshida to eat her out, while Eric(a) eats Yoshida out. No longer aware of anything else, the Necromancer will use them as her own sex toys until they reawaken.