A few considerations and thoughts to put out there, as I have read some wild speculations, some guesses, some good ones and also some things that put simply are not in the game.
This will not be comprehensive or exhaustive in any way and be based on me skimming through a bunch of comments.
Facts: Things that are clearly stated in the game or that are background elements to the game but may have remained not expressed.
Headcanon: my own speculations, interpretations, preferences, that are NOT to be considered Gospel even though I am involved in the making of this game. Any and all of this might be entirely subverted or negated in future developments. it IS true that some of my headcanons might become one day actual canon if it is a direction that the boss likes as well, but there is neither a guarantee nor an obligation in that direction.
Rabiah has had Nadia sometimes after MC became involved with Zahra. Was he still the father? it is unclear and remains not expressed nor insinuated. you can make up your own mind about this until the game tells you otherwise. for now, any opinion is valid.
The twins: Rabiah is not one of the twins.. the twins are her babies.. they are still nursing and will not appear on screen or anywhere else. It is not explicitly stated, but there is a heavy implication that Omar is the father and that Rabiah has settled for him several years ago and has become a bit more "boring" until moving back to Zeta.
Nadia has the biggest "you're not my dad" energy towards Omar and it is explicitly stated that she knows he is in fact not.
My personal take is that Rabiah probably was married to someone else before Omar, and that person would be the father. Rabiah has missed the babymaking train with MC when he went after her sister. I am personally not bothered either way, but I would think that MC would have been too busy enjoying his new wife and building his career to have time for the ex with the bad attitude.
MC and Zahra's finances: the compound they live in has been developed over time and is still susceptible to changes. this is to simplify the job for the boss and the narrative for everybody. At present, MC is rich and renowned. He is part of the 1%, so to speak. This is the result of 3 decades of work. Their approximate ages are: late forties to early fifties for him, around 40 for her. there's some discretion for you to give them whatever age works best in your mind.
MC had started going on explorations already, by the time he tied the knot with Zahra, and he might have gotten lucky a few times and struck gold. In other words, monetary concerns are background noise to the life of the MC, and even early on they were not hurting.
It should be noted that the spouses of Zetans who were in service of the city, specifically in the military, on missions and, one could argue, also Stalkers on official business, are awarded protection by the Palace.
So "if" Zahra had been financially unstable during the early years, as long as MC was doing jobs for the Queen of the day, she would have been provided for, granting some measure of stability.
Zahra's fertility over the last 18 years is an interesting topic. I would submit that they might just have decided that 2 girls was enough for them.. and that they took measures to ensure that things would stay that way. you may make up your own reasons or methods.
My personal take is that maybe the pregnancy with Ain was complicated and dangerous, and that Zahra, in agreement with MC, decided to use a fantasy version of the pill to stop it from happening again. maybe now she's reached an age where pregnancy is just extremely unlikely so she has stopped taking the pill.. maybe she's become barren in time.. maybe she will pop out a surprise baby. it is not particularly interesting to me, but there are dozens of ways or reasons for this to be perfectly justifiable.
likewise, maybe the multiple experiences in various Zones have left the MC able to procreate only in special situations, or maybe on command... or maybe the reason that they haven't had any more kids or that he hasn't inseminated every girl presently in the game, is that it's a game and if there's a pregnancy kink to explore, it probably should be done with active participation and choice in the matter on the part of the players.
these are all thoughts of right now, do not hold me to any of this.
The hit and miss realistic approach to historical impossibilities or scientific inaccuracies or technological implausibilities is due to the nature of the game, the limited resources when it comes to show things (there are only so many ways you can portray post-apocalyptic reality without getting lost in the weeds of what can't be, especially this limitation is true if we consider the existing library of 3D models and tools. none of us are actually 3D artists who draw objects from scratch).
What we do toss in is to reflect the movies and games and other forms of artistic expressions that are inspiration to the game.. and yes, sometimes people who have even only surface level knowledge of a specific topic will be pulled out of the narrative because of such an inconsistency.
Having been around horses a lot, I experience the same when I look at the charge of the Rohirrim at the siege of Helm's Deep. those horses should have rolled all over each other, on such a steep incline... and yet I am perfectly fine with Elves, Ents, Nazguls and so on.
A solid amount of handwaving is demanded to be able to enjoy this game. we try our best to make things somewhat believable and my personal interest goes into making this as coherent and logical as possible, but it will never work if you look too closely...
I believe that our best shot is to make believable characters have believable and meaningful relationships and reactions to one another (within the logic of occasionally going all porny for the sake of porn).
P.S. don't come at me for technical issues. I know less than you about them.
Thank you for your inside insight on what is, is not, and may be. It is INCREDIBLY helpful!
I had not realized our boy was THAT rich: that certainly changes things a bit. Is he that rich specifically because he focuses on working for the queen? Guessing stalkers who don't focus working for the queen w/n get the Queen's protection, at least.
Did the council who created Zeta also create the profession of stalker along with it, in order to collect technology from out in the field (where others could otherwise get ahold of it), and bring it specifically to them? Or is it more like the profession just sort of free-market-opportunity created itself and the Zetan council recognized it's key role and made aggressive use of it?
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Regarding pregnancy game mechanics..
The game DOES already have an on-off switch for pregnancy under the active control of the players: it's called "pulling out".
One real world caveat on pulling out is that it doesn't always work to prevent pregnancy: precvm can actually get a woman pregnant, and you might also blow a bit before you fully pull out, as well. More, if you cvm in her ass or on her belly, and any of it slides onto on her labial lips, she can actually still get pregnant if any of that seed seeps into her (potential bad news for Shani taking her dad's loads on her belly after workouts: tho she covers up her pvssy w/her hand, some cvm could have slid down in there, anyway). That said, I would guess a game would tend to ignore such uncertainty as it would be damn inconvenient.
Also, not sure what time scale the game is going to provide, but it usually takes several months (in a more traditional society without advanced technology) for a woman to know she's pregnant. Not sure the game's ever going to get to that point.
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Regarding Zahra having fewer than the expected number of children, despite her and the MC believing that Rabia even having THREE kids in LESS time was too few....
If the main character is sterile, that really screws the player over because it takes a lot of the punch out of playing him. I don't think it would be wise to go there.
The MC being potent only occasionally or on command is a little odd, and I think we would have heard him thinking about that in his internal monologue.
For a certainty we know he at least THINKS he's putting babies in there, because he fully believes Talia's later 6 kids were his (actually not likely all are, if Talia really is the palace cvmdump the queen says she is), and the Lyrah humiliation has him at least SAY he can get her with child.
(BTW... Please tell the devs here is at least one vote for Lyrah ACTUALLY being pregnant from him: pretty please, with Lyrah's popped cherry on top. It is what makes her humiliation REALLY hit: that she might be pregnant from the MC, who she was pretending to hate and be offended by. It would also be so nice if eventually, if Lyrah is actually pregnant and gets used to servicing the MC whenever he strolls by, she realizes she just wants to be his concubine (harem increase); even though it would be her again saying she wants to quit the guard, the queen might actually find the full-submission outcome amusing. Obviously if Lyrah went the scavenger route, the MC has already potentially blown his load in her, and has offered for her to live with him.)
I personally like your headcanon that (IF we are going to explain why Zahra had so few children) it is specifically the Ain pregnancy that either rendered Zahra infertile or convinced her to take "the pill", thereafter. The name Ain, in addition to meaning " eye" (appropriate, as Ain's eyes are so intense), also means "precious": if they thought she was going to be their last kid together (and if she had been a difficult pregnancy and they weren't sure they were even going to be able to have her but she proved a miracle baby and popped out healthy, anyway); they might very well have named her "precious", as a result. The Ain pregnancy being difficult would also fit w/the family endlessly sighing that the girl's a troublemaker: in this case even from before birth (although they might not have told her that, to keep her from feeling guilty). I also wonder if Ain's short stature might have been related to her being a difficult pregnancy as she shorter than everyone in her family. Obviously supposedly she's 18 or higher, so her height should be what it will always be going forward. That said, the character artwork for the game certainly looks more like they go with the pre-industrial standards of adulthood, which place it a few years earlier, in which case she could theoretically keep growing taller, but we're going to ignore that possibility since officially everyone is 18 or higher, for the usual reasons.
BTW, we already know that Zahra uses lots of little tricks to keep herself looking young, and she really does seem to have a lot of " womanly wisdom" in general, so it would make sense that she'd be well schooled in such matters, and obviously the MC is more than rich enough to afford whatever treatments are necessary.
Also.. (fertility questions aside).. Please tell the devs that (at least imo) THEY GOT ZAHRA ABSOLUTELY PERFECT as is: don't change her! She's amazing! She's the perfect woman, exactly how she should be: incredibly wise, disciplined, and loyal, and her relationship with her husband is also spot on; the absolute respect they have for each other and each other's roles is just flat-out RIGHT. You also can hear the different foci each has, with Zahra being incredibly family focused (such as her utter terror upon hearing Lord Abrax knows about Shani). The devs really stuck the landing on the traditional ideal husband and wife dynamic. I mean, aside from the couple fvcking their own kids: THAT'S... Errr.. Yeah. Just tell the devs thanks for getting her character so incredibly right: this is one player who likes to see such things done properly.