Marceline has shown to be the closest friend Finn has aside from Jake. In the episode "Go With Me", after stating she didn't want to date him, that her condition was no tongue. Now why would you make a point of no tongue, yet nothing saying he couldn't kiss her otherwise? That implies they could kiss which resulted in a confused shocked Finn answering "Ye-what?" With how she teases Finn, surely if it was a joke she would have laughed or enjoyed the moment of his confusion.