History's Best Friends
Very rare1.5%ObjectiveOptionalSingle-player
Sleep with your best friend while having the "Ritualized Friendship" cultural tradition
Needs Fate of Iberia (Ritualized Friendship is an Iberian cultural tradition). The wiki setup: Ibn-Marwan of Badajoz in the 867 Struggle bookmark starts friends with Emir Sa'dun of Burtughal, who will usually accept an Oath of True Friendship - upgrading him to Best Friend. Set the Sexuality Distribution game rule accordingly at setup (it doesn't disable achievements), swear the oath, then run the Seduce scheme on your best friend. History remembers them fondly.