You're not as far as I thought. Pema's scene only triggers after you had Korra's anal scene. Don't worry, the loli part is optional.
If you have visited Varricks factory, you have to visit Azula next to say goodbye. Always try to speak with Tenzin at night to progress the story. If he asks you if you had sex with Azula, Korra's anal scene should start the next day in her room. (She asks Jinora to leave.)
Thanks. Yes, Varrick's factory scene was a while ago ... I'll go back and visit Azula again.
The problem is that in this part at least, there's no clue that you need to repeat things. Usually in a game, if you've done the scene, it doesn't need repeating. I didn't realize I needed to keep doing it over and over again.
At other parts in the game, you'll get hints like "I don't think I'll go to sleep yet, I still have something to do." A hint like "I should make sure I have said goodbye to everyone" or something would help.
I'll go try that now. Hope it works!
EDIT: OK, so I went to Azula, and had sex with her again for the who knows how many times ... there was absolutely nothing different about this time. She didn't say goodbye or anything. So I thought "Well, I guess that wasn't it" and then I went back to Pema and suddenly she has a new condom. The game progressed and now I'm heading to a bookstore.
What finally triggered it was apparently the last visit to Azula but geez, game designers, maybe a hint after I've visited everyone what seems like a dozen times with the exact same scene each time?