One of Crusader Kings III's most-requested features will be arriving with update 1.5, alongside the Royal Court DLC. Same-sex marriages, which up till now have only been possible by using mods, will be added via a new custom rule in the grand strategy game's menu. Due to how Crusader Kings III's game logic works, and all the rules in place about feudal marriage and succession, this wound up being a complicated addition to the game, but it's finally arriving with the next free update.
Currently, you have the option at the outset of a Crusader Kings III campaign to set custom rules for same-sex relations: by default, each culture has a specific view on same-sex relationships - usually, a negative one. However, you can change things so that all faiths in the game accept same-sex relations, which allows you to pursue those throughout the campaign. However, you currently need mods to make these relationships official.
With update 1.5, a new game rule can be adjusted so that same-sex marriages are possible without any mods.
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