I think it's the variable called persistent.OttoFuck, but that's getting marked as True just fine once you've seen it during the story.
It's just that there's nothing related to the variable itself in the screens.rpy file.
I'm guessing there should be an entry checking that variable related to CS_OttoDocFuck or something, since that's the scene that's not in the gallery.
Also the gui/SexScenes folder that has all the Scenes gallery icons doesn't have the related png. (I'm guessing it'd be Otto02_idle and Otto02_hover)