They're not difficult, but if someone gonna try that, paradox will immediately release update with gay marriages
I don't really understand your point? There's no guarantee of it happening right away aside from mere supposition, and even if it ended up happening, that someone was industrious enough to get a workaround to work... it would be a good thing anyway? We'd get what we were asking for either way, except it wouldn't be trying to get viable solutions by modifying a million things anymore, just a simple toggle.
It's not historically accurate isn't a good reason because neither is a Hellenistic empire out of India taking over the known world in 1307.
If anything, the game skips over a few things in favour of a more contemporary, nuclear view of marriage and social relationships. I can only recommend reading, for the medieval Christian world,
Same‐Sex Couples Creating Households in Old Regime France: The Uses of the Affrèrement by Allan A. Tulchin; and
Same-Sex Unions In Premodern Europe by John Boswell. I'm still gathering documentation on the Islamic World: I gather lesbian unions were a thing in 17th to early 20th century Iran, but I'd like to see when exactly was the practice established, and if there was more than just that one. I'd like to check for India and Tibet too.
Something to take into account when considering romantic relationships in Medieval Times, especially in the Christian world, is the fact it was not the way people thought of it at that time. As Karma Lochrie states in
The Cambridge Companion To Lesbian Litterature,
"The sexual landscape within which same-sex desire – male and female – was understood in the Middle Ages was not defined by heterosexuality, that is, the prevailing “normative organization of the world” in which love and sex between men and women serves as the standard and ideal of sociosexual relations, as well as the principle of cultural organization, law, and citizenship."
Instead, she says, sex was organized on what she calls a "continuum", with virginity considered the best thing, sex within an opposite-sex marriage good, and various practices within said marriage shunned or criminalized. What was considered the basis of an ideal relationship was instead friendship, and indeed it is the most represented form in late medieval literature (see Percival embracing Gawain passionately in
Le Conte du Graal, for instance). What people did in bed was not that important to Middle Age people; those passionate frienships were instead the basis of social relationships between two people.
This brings me to the concept of
Affrèrement: it is not unlike modern-day Pacs in France,
i.e. a legal union between two people (or more at the time, which lead to
a lot of non-nuclear family structures) that allows them to live under the same roof and share the same resources, along with inheriting what the other(s) possessed at time of their death - and sometimes being buried alongside each other. It was common in the late Middle Age and mid-Renaissance in Provence, Midi and Italy. In the Orthodox world,
adelphopoesis was of a similar nature. When two single men from different families did it, was it what we would call "gay marriage"? No, simply because they hadn't invented the term yet. Was it at least sexual? Not always,
but at least sometimes. And that sometimes is more than enough for our purposes.
Why? Because Crusader Kings 3's concept of marriage is not merely representing sexual relationships between spouses, but also working ones. Your spouse, in the game, is not merely the person you make babies with. They are a fully fledged member of your council; they give you bonuses to stats, to prestige and piety. They share your life in more ways than one. The issue with a lot of responses in this thread account to the idea that marriage has to be sexual, but we've established that in the Christian world, marriage-like unions between men -- and less often women in the case of
Affrèrement -- were somewhat common; and passionate relationships between men and between women were dime a dozen. Most of the time they weren't what we would call "homosexual relationships" or even "gay marriages", but that's alright, because for our intent and purposes, it does not matter.
It would be anachronistic to call them that, but hey, lo and behold, the game does not care. It already has the mechanics to justify such a union. Should it be integrated in another way than modding? Maybe not, at least not with the way cultures and religions work right now: there are too many different practices on a long span of time (and don't get me started on the modern day gender binary implemented in there -- hello southern Asia~). But it wouldn't necessarily be ahistorical.
So uh, sorry for the overly long post, gonna go find more stuff about the Islamic world, sub-Saharan Africa and southern Asia now~