That just means someone in charge made a bad choice during planning and either no dev or designer ever dared to speak up against it or many of them supported said bad choice.
Why is it a bad choice? A game set in middle age Europe has no more reason to foresee needing the ability to mod gay marriage into it than accommodating modding cell phones into it. It's not an insignificant amount for work just to include an anachronism.
Have you played this game and seen how much you can customize it?
What's an anachronism in a sea of many others, if you wish to have them in your personal save games. And if you don't, you stick to whatever is historically accurate.
Not having the foresight to be aware of this matter and the issues that come with it(going as far to be be especially exclusive) as a game studio in 2020 is what they should think about the next time they set out to make a game like this.
Given a preference I'd rather keep "but it's current year" out of the considerations of the devs of games I play. It's not a role playing game. It's not Tomodachi Life. They owe us nothing except what they advertise.
And the game engine is from 2007 which likely built in many of these historically accurate constraints to reduce scope. Remember until recently this much more core functionality was broken; "Fixed looting not making you hostile to looted country if you’re a vassal"
As someone who's lost many nights to CK3, Elite Dangerous, and not Star Citizen I'm grateful they released a limited very enjoyable game that simulates the core elements of the concept, and I hope they enhance it with native support for Gay Marriage in the future.
"Just" made a bad choice? It's clearly an historically accurate, and _simplifying_ assumption, which you dismiss. The fact that they include a scripting language needn't obligate them to broaden the scope of their project. It's theirs, and they needn't pretend to cater to anyone they don't wish to, especially if it isn't a priority. They can make whatever they want, as can you.