Yeah lets leave those ideas to the realms of modding, shall we?
Project Zomboid is *not* trying to be the most realistic game possible. PZ is a story generator, following the doomed experience of your player character. Realism factors in at a lot of places, but TIS has, on numerous occasions, opted to go for more game-y systems over realistic behaviour because the truth is: Realism isn't fun. It's possible to enjoy realism, but it's not fun if you take it to those frankly ridiculous extremes.
As for the idea of a "reproductive system": There's so many reasons why this is a bad idea that listing them all is impossible, but here are a few anyways:
Ratings: A lot of countries restrict the sale of games with sexual content. Either they get banned entirely (Australia is a common example for this) or their purchase restricted to platforms with age verification. Having the content be hidden behind a toggled setting is not good enough to satisfy those rating boards.
Player behaviour: Players are assholes. You have to always keep that in mind when designing games with multiplayer. Every feature you add needs to be thought out in particular regard to how easily it is exploited to ruin other player's fun. I don't think I need to elaborate too much on how a "reproduction" system can be used to abuse players.
It's pointless: On the time scale the game runs on, seeing any possible child being born is already unlikely. Few players survive 9 months, and even fewer actually play that long before rerolling out of boredom. Let alone the decade+ it would take for that child being anything more than an annoying burden.
Violence & Children: This one plays a bit into the ratings issue as well. Having children in this game also automatically implies the possibility of violence against children, and death of children. This is something that a LOT of people are not comfortable with, and that most ratings boards do not look kindly on. (Ever wonder why if there are kids in a violent game, they're almost always immortal?)
There's some other points I won't elaborate on, it doesn't take much thought to come up with more reasons why this isn't a good idea.