Before becoming consort to the chosen king, Prompto was well known for giving the best blow jobs in the city. Absolutely legendary for it. Noctis, either from a lot of flirtatious references/jokes or Prompto outright admitting it, finds out. It turns him on immensely and he wants details.

+Humiliation kink
++Videos of Prompto's 'talents' exist
++++Noct inviting a third (or more) to watch Prompto go down on (pref Gladio, but really anyone)