I personally wouldn't want a seemingly historically accurate (ish) game to present the churches of the age as being okay with same sex marriage when they weren't. Although being able to toggle it on in the settings would be cool.
I'm guessing it's just more or less something they didn't get around to implementing. Perhaps they wanted to tackle it in a future patch or DLC (I can see this happening if turning this on somehow led to buggy behavior in other areas)
https://theconversation.com/a-thousand-years-ago-the-catholi...
It is as though one attempt to translate the ranks of entirely different historical military structures.
Though, such is also often done, such as the “Kings” of Rome which were elected and had a co-ruler.
In any case, one might for instance argue that a male Japanese samurai in 1800 could be “bound” to both a male and a female at the same time, but only one of each, but only the latter union is traditionally translated as “marriage” for “reasons” even though the former comes no less close. In act, the former union was typically more so one of love, and the latter more so one of business.
No Christian cultures, obviously.
Homosexual activity cannot result in biological children so there was really never any society that considered anything like homosexual marriage until our own. The main reason for marriage was to control sexual unions and the children who might result from them. In societies which had marriage (many had no such concept) homosexual marriage didn't make much sense.
The wikipedia article on historical same-sex unions mentions that some cultures had rituals around cementing homosexual relationships but those wouldn't be considered the same as marriage, they'd be more like any pre-sexual / pre-relationship rituals. Marriage typically is about the sharing of finances, responsibilities, the raising and rights of children, etc. And none of the examples in the wikipedia article on historical same-sex unions concerned any of these.
I'm not sure that the points you're making about shared finances are particularly important, or even true, based on this. But I'm not a historian.
In this account of the relationship between two men they are termed in quotes as "husband" and "wife" to imply sarcasm . They were not literally considered with seriousness to be married by their society the way we consider men to be able to do to each other now.
Marriage, in societies where it existed often served two functions: binding families and providing for the legal rights of children. Unions of lovers did not do either of these.
Many ritualized unions of a same-sex variety in history were of the latter type and those of the former would not likely have been seriously recognized by the outward society. There isn't a strong record of that anyways.
Interestingly, the vast majority of countries that now permit gay marriage have Christian histories.