At a recent public event, the actress, Olivia Olson, who voices Marceline on the show Adventure Time, revealed it is official canon that Princess Bubblegum and Marceline used to date. She learned this in a conversation with Pendleton Ward, the show's creator. Here's the video evidence: 

In celebration of the news, here is awesome fan art celebrating Princess Bubblegum and Marceline's relationship:

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