I would also like to advocating changing CK3 to allow gay marriages, in modding, at minimum.
I do not think it would even be especially reasonable if the base game included gay marriage. This may not be historically accurate for the time period, but I think it could make sense for player-created religions. If I create a religion featuring both the gender equality doctrine, and which is accepting of same-sex relations, I think it could make sense in context to permit same-sex marriage in that religion. If a gay man creates the empire of Scandanavia via force, and then reforms the Asatru faith, and becomes its High Priest who, exactly, is going to stop him from marrying his lover?
It is possible in CK3's base game to create a religion which allows, or even encourages rulers to marry four of their own daughters, while it is not possible to create a religion which allows a man to marry another man, or a woman to marry another woman. As far as I am aware, neither possibility is especially accurate for any offshoot of Christianity within the time period. I do not see why historical realism should prevent the creation of an offshoot of Christianity (or another religion) which does permit same-sex marriages. If I am permitted to look to more recent times in history, several currently extant Christian sects permit gay marriage. To the best of my knowledge, no Christian sect currently, or previously existing permits plural father-daughter marriages.
I was excited to see CK3 normalizing non-heterosexual characters in the medieval time period, even asexual people (almost never represented in video games). I think it is disappointing that CK3 is also designed to restrict the possibilities of permitting gay marriage in actual game, only allowing unofficial affairs. While I appreciate that Paradox games depict historical situations, I always thought the entertainment value of Paradox games was in that they allowed the depiction of alternative histories, even some alternative possibilities that would be implausible in the real world, such as re-creating the Western Roman Empire, along with re-instating Hellenism as its state religion, in the year 1300.
I have a less strong opinion on the issue of implementing same-sex pregnancies in CK3, and I see no real rationale for it in the base game. However, I am aware that people clearly desired it in modded CK2, and I feel that CK3 should permit same-sex pregnancies in mods as well.
I would also like to see the return of the set_gender command from CK2, especially as the game lacks a character creator. I think it would be interesting to allow people to explore playing as say, Queen Louisa the second, of Italy. It would help add variety to the initially playable characters.
It would be interesting to also add a command to set sexual orientation. I was excited to get a chance to play as an asexual character in CK3, because I almost never see asexual people, like myself, represented in video games.
In 867, there are, a total of 6 Asexual top-level characters (browsing vassals in the map at the start of the game is hard), according to my best attempt to examine the entire map (it is not possible to use the character finder to search by sexual orientation). Of these, the most powerful is the Duke of Nepal, with the other 5 independent asexuals being counts. Interestingly, none of them follow an Abrahamic faith. Introducing a command to set sexual orientation would allow people to more easily explore the implications that a ruler's sexuality may have on the game, in a variety of settings.
In general, it is disappointing to see that there are any aspects of CK3, as a game, which are more restrictive on queer experiences in the game than in CK2, even in mods which change large aspects of the gameplay. I was excited to see that CK3 intended to be more inclusive of queer perspectives, and it is a let down to find out that it restricts the potential to queer the game via modding.