If you are reading this comment your parents will die within 5 years. To undo this curse you need to copy this and paste it on 5 other manga. i'm so sorry
When I reached the end, I was confused.. As to what "Proof" they were talking about. The I went back to the convo between Levi and Erwin. If I'm not mistaken, they wanted to try proving that Eren wouldn't rebel and will be under control of Levi. Hence the sex