Kurt leaves for a while.
Sebastian talks to Blaine about all the things that he can do and stuff that Kurt can't. What they do not know is that Kurt can hear them, and Kurt can see that Blaine is aroused.
Then comes possessive Kurt to extract Blaine from Sebastian.
Kurt gets jealous and demands why Blaine is aroused. He says that he thought of Kurt doing those things to him and not Seb.