Paradox Interactive removed this feature last month, citing gameplay issues.
According to the enthusiast, it is possible to marry two characters of the same gender only through the game engine, so such modding is considered an illegal operation and can provoke new errors.
Waffle Ironer has released a fan patch that contains several other changes besides same-sex marriage. After the patch was published, Paradox blocked the modder and deleted all posts that mentioned his work.
The fan explained that he understands the developers' decision and asked people not to pester them with questions. So far, Paradox hasn't figured out how to add gay marriage so that it doesn't harm the game, but it is still working on it.