No, she was straight, and the animation team had originally dropped the idea that PB and Marceline were lesbians.
In his panel interview from 2012 Ward hints that Rebecca Sugar was trying to push for Marceline and Princess Bubblegum to have once been in a lesbian relationship, but doesn't use those words or clarify specifically on what Sugars "plans" were, or if he's saying they were or weren't in a relationship or just friends, only that he doesn't want to spoil anything for fans by telling them what their backstory actually is.
As a result of the discussion going on behind the scenes on what PB and Marceline's prior relationship entailed, a company that worked for the animators on the show were fired by Cartoon Network when they said online that PB and Marceline were once a couple, causing Adam Muto to state that they had been fired for stating possible "subtext" as fact.
By 2014 during an interview, Olivia Olson was asked by a fan if Princess Bubblegum and Marceline had been lovers in the past. She says that Ward told her that the team discussed the idea, but that they ultimately dropped it. Olson says joking that she thinks they were once lovers because of all the "creepy fan art" that people make, but that CN wouldn't let them anyway, because it would hurt the show over seas:https://www.youtube.com/watch?v=_EleCNWfwTY
This resulted in people saying Marceline and PB were 100% lesbian lovers and writing articles on websites about it, which caused Olson to immediately go on twitter and clarify that she was talking out her ass at the panel and none of what she said was official. This however angered a certain segment of the fan base who were obsessed with the shipping of PB and Marceline, who took to whining and bulling Olson into deleting her retraction statement.
It was pretty clear by this point, that they kicked around the idea of PB and Marceline being exes, and had dropped it for various reasons leaving it vague and up to interpretation.