It's a one-byte modification if you want just to remove the check for mixed genders upon marriage. If you want to implement as proper game feature - as a selectable option - then you need to do a bunch more things; you need to model the conditions in which that same-sex marriage would be permitted (even if it'd be enabled in the game as such, there would be character-faith-related conditions), you need to model how every other character would treat that marriage (a bunch of conditional modifiers depending on the attributes of that other character), you need to decide how it interacts with polygamy if someone has spouses of both genders; you need to adjust for any impacts on the many inheritance models (after all, the main gameplay purpose of marriage as opposed to just lovers is the effect on inheritance); you need to adjust any events and event text (and translations) where it does not make sense any more; you'd likely need to change the treatment of bastards if one is born in a woman-woman marriage and you need to treat edge cases such as regency if both spouses are pregnant (from other partners) at the same time; etc, etc.
It's far more than a one-byte modification.
