Honestly I am very shocked it isn't available in the game because of one simple mechanic that IS in the game. Infertility. This game is supposed have a multi-generational aspect to it but if you happen to marry a spouse who is infertile or possibly becomes infertile due to a late marriage, how the hell are you suppose to have any sort of multi-generational gameplay?
I mean I am all for realism but if you want the realism of infertility, you have to add alternative ways to produce a heir. Historically, Rules often divorced if their wives did not conceive in a reasonable amount of time. Alternatively they would sire bastards and if not official heir came about, would adopt their bastards into the royal family. Other rules had harems or multiple consorts in addition to their official wives. A heir is a must and you must do what it took to ensure the line of succession. That is the reality of it.
So if we are going to have infertility, we have to have alternate way to produce a heir. We need to be able to divorce and find a new, fertile wife or be able to have bastards with our companions (I like this idea fyi) or occasional have a bastard show up in a city you visited at some time in the past or multiples wives or something to ensure you have at least one heir before you die. The other option is to remove infertility (and maybe even stillbirth) to ensure a heir is conceived and born.