(just empty talk in this paragraph)
So, browsing random videos on youtube I've stumbled onto a Kabaneri AMV, which in turn reminded me of a hentai doujinshi I've read on it some time ago, where Mumei defeated the kabane by fucking them to exhaustion, or at least that's what is seemed like, as it was in Japanese (or maybe even in Chinese, I don't remember for sure), so I don't know the story exactly, but there was a lot of extreme sex, like with prolapsed womb and stuff, though the somewhat grotesque artstyle wasn't exactly to my liking.
(end of empty talk)

Anyway, remembering that thing gave me an idea.
The basics of the idea were: a zombie apocalypse where males become zombies, females survive, zombies rape survivors, but there are also young and pretty heroic girls who fight zombies by fucking them to exhaustion.
However as I tried to write it down and think of how such a world can actually work, I came up with the following.
So, there is a zombie virus. Well, not really ‘zombie’ as it has nothing to do with living dead. In fact, most of humanity have just died from the virus and is not going to raise from their graves. The infection didn’t spread through bites, it spread through air, and have spread so quickly that everyone were infected before anyone could do anything about it. There appeared to exist an immunity though, and not one, but two kinds of immunity. One protects from any effect of the virus (I’m going to call them survivors), the other keeps you from dying, but turns you into what I’m going to call a zombie. They are still living beings needing food and capable of procreation, but they are not sentient humans anymore, they become mindless monsters driven only by instincts. Also, survivors are mostly female, while zombies are 50/50 male and female. But female zombies usually stay at home and tend to their zombie children. You can even say they’re peaceful, at least as long as you don’t come to them first. Male zombies are different. They search around for food, mostly for meat. Being physiologically human they are capable of eating plant food too, but they don’t like it and would use as last resort. Usually they get food by hunting, maybe capable of using sticks and stones they pick up, but probably too primitive for anything more advanced (like at least to sharpen one stone with another). Of course, they are nowhere near sentient enough for agriculture and ranching. Instead they have probably developed beastly speed, agility, reflexes, ferocity and survivability (they should, after all, pose a threat to survivors) and hunt both animals and survivors who ventured into the wild. But sometimes (especially when suffering from shortage of food, like in winter) zombies would gather in large groups and attack survivors’ settlements.
Now, normally they would see survivors for food following their prey drive. But when they see a young and attractive girl (and most survivors are female) another instinct kicks in, they forget about everything and would keep raping her to death and probably some more. At least, that's what normally happens. However, if such a woman manages to survive (say, her friends killed the zombies raping her before she died from it) and bears zombie’s child, if it happens to by a girl, such a girl would become a nymph (and if it happens to be a boy, then no luck, he would be inviable). Nymphs are girls with even more insane sex drive then zombies themselves, or maybe it would be better to say, sex stamina. They can take a lot of rough sex and even enjoy it, but since unlike the infected men they are still normal sentient humans, their sex drive doesn't control them, they control and put it to use instead. Namely, they fight the zombies by having sex with them, even able to seize the initiative, until the zombies, unable to suppress their sex drive, exhaust their own selves to death (or at least lose consciousness after which either easily killed with conventional methods or left so weak they have little chance of surviving anyway). In order to attract zombies nymphs wear little to no clothes. Certainly nothing to be in the way of sex. So no underwear and a shortest skirt possible if any. They would not only dress, or rather undress, so for 'combat', but for daily life too, proudly wearing nothing / almost nothing as a sign of their belonging to the honored cast of nymphs.
The most obvious downside of being a nymph is that they are not capable of bearing children. So nymphs can’t make more nymphs.
Another downside is that they get used to rough sex and might end up recklessly 'attack' more and more dangerous groups of zombies in search for satisfaction, and might end up swallowing more then they can chew, so to say, that is, their stamina is high but not unlimited, and they risk being raped to death just like normal girls, it would just take more/stronger zombies to do that. Not to say they have no hope finding satisfaction in sex with humans.
Yet another downside is they have to be young and attractive to properly trigger the zombies' sex drive, but it's not like they don't age. Ideally they try their best to keep their beauty and then retire from being a nymph and live the rest of her live as an honored ex-hero. But it is not unimaginable for a former nymph to feel so unsatisfied, she would end up seeking sex with zombies again, even though they are more likely to just eat her, and even if they choose to fuck her, she's not as young and healthy as used to be and easier to fuck to death.
As for the male survivors, being few in numbers and vital for procreation, no wonder they along with nymphs form the elite of the society. Or at least some of societies, I imagine there can also be those that hold such men as sex slaves forced to ensure procreation, and there can even be ones that treat nymphs badly. But speaking of the former type of societies, where both immune men and nymphs form the elite, I imagine it might be customary for immune men and nymphs to marry each other, even though it would be more of a symbolic political union. Even if they actually fall in love with each other, the nymph still has a duty to fight zombies with sex and the man has duty to impregnates other girls in the settlement, he also has no hope of satisfying the nymph or impregnating her. All he can do is hope she would be kind enough to satisfy him without asking anything in return, which she would be able to do masterfully with all her skill and experience.

Genetics
The whole immunity thing is tied to a certain gene, let’s call it ‘A’. It has three alleles:
A-1 – die from the virus, dominant;
a-2 – survive the virus, recessive;
a-3 – become a zombie, recessive.

Interactions:
A-1 dominates over either a-2 or a-3;
a-2 + a-3 gives a nymph.

So:
People with at least one A-1 (most of the world’s population) die from the virus.
People with two a-2 become survivors.
People with two a-3 become zombies.
Females with both a-2 and a-3 become nymphs.
Males with both a-2 and a-3 die from the virus (or from the gene combination itself, doesn’t really matter).

Side effects:
Male survivors (two ‘a-2’) have very low chance to father a son, but still normal chance to father a daughter (let’s say some condition affecting Y sperms, but not X ones). This is why, aside from generally being rare, the two ‘a-2’ combination is even more rare among males (hopefully it works that way). It also assures the low male to female ratio among survivors will persist.
Female survivors (two ‘a-2’) suffer no side effects.
Nymphs (‘a-2’ and ‘a-3’) survive and have increased sex drive (hence nymphs) but cannot bear children.
Female zombies (two ‘a-3’) can only bear zombie children.

Inheritance:
A child of two non-nymph survivors is always a non-nymph survivor (99% girl, 1% boy).
A child of two zombies is always a zombie (50% girl, 50% boy).
A child of a female survivor and male zombie is 50% nymph, 50% inviable boy.

_________________________________

Actually, this is the best I could do with genetics, but maybe someone can do better.
Ideally, I would like a scheme where:
- Rate of male to female among survivors is very low and they can't just fix it by eugenics;
- Rate of male to female among zombies doesn't need to be exactly 50/50, but must be capable of maintaining their population (so a reverse from survivors: 1 female for 100 males, would not work);
- Zombie male + survivor female have some chance of having a nymph, but not a big one;
- Male would be nymphs can not be used to breed nymphs (as I see it, to achieve that they should be either incapable of having children, or just inviable in the first place, but if there is another solution, then it can work too);
- Nymphs can bear children;
- Nymphs + male zombies have a substantial chance of having a nymph (more than zombie male + survivor female);
- Zombie females cannot be used to breed nymphs (I had to make them unable to bear non-zombie children at all to achieve that, but if there is another solution, then it can work too);
- Basically, there is no way to mass produce nymphs, best chance is by sex with zombies;
- Two survivors should normally give birth to a survivor and two zombies to a zombie; the might be slight chances of a zombie being born from survivors or vice versa, or even a slight chance of a nymph being born, but that should be slight enough so that nymphs must seek sex with zombies in order to effectively make more nymphs;
- Same goes for nymphs + male survivors (must give birth to survivors, at best zombies, but chance of nymph, if exists, should be too little to be of use);
- In the most perfect ideal there should be incentive for survivors to not eradicate zombies entirely.

I was also considering an option where male survivors either don't exist or incapable of having children, and then the survivors actually need zombies to procreate. But I couldn't resist the temptation of a settlement with a male leader where half of the settlement are his sisters (daughters of his predecessor) and half are his daughters from those sisters, and he keeps fucking them all for the sake of humanity.
But now that I think of it, such a scenario would probably be closest to what I imagined initially. The survivors are only females and the zombies are only males. Survivors need zombies in order to procreate which is why they don't kill the baby when a male (which means zombie) is born and let it go, probably even care of him until he can do so on his own. This allows both parties to keep living. As for nymphs, they aren't born from sex with zombies, but just randomly, or it might be that it isn't as much genetic int he first place as personality + training. Anyway they fuck zombies to exhaustion, but don't kill them. They do so to protect the other girls from being raped to death, as zombies aren't capable of self-control, and also to procure semen and then share it with other girls.
Though in either scenario, now that I think of it, there doesn't seem to be a logic to why they don't just capture some zombies and get semen from them for either nymphs (in one scenario) or just procreation (in the other).

Ah, anyway. I don't know what would be the best thing to do with that idea. So do with it what you want.
As with all my ideas, do with it whatever you want, though I hope for some credit in case you actually use it, and I keep the right to use it too in case I suddenly have inspiration to do so, even if it's after you've already written a whole series of books on it which got a TV adaptation and you made a fortune and world fame on it.