I mean, even I could tell that. Bubblegum and Marceline rarely interact. I mean, I understand as friends but the whole thing was weird. It's just weird.
Although I might just be salty that it wasn't Finn for either one of them.
Do you want to know something saltier? If Pendleton Ward would not have left the series, Finn and Bonnibel would ended together, because Finn is based in him and Bonnibel in his wife.
The two of them have had several moments together for the past few seasons (especially the Stakes miniseries back in 2015) so it really wasn't that surprising.
Here's what Adam Muto said about their relationship in an interview:
Marceline and Princess Bubblegum’s relationship was one of the most meaningful parts of the show, in terms of representation. What does the positive reaction to their pairing mean to you, and how did it influence their beautiful ending?
It felt like that was another question. It is set up enough by now that it needs a conclusion to feel satisfying. We can’t say what their life will be like from then on forever, but we can show where their relationship is now as opposed to where it was a couple years ago. Marceline is the adolescent forever, and Princess Bubblegum is the workaholic who doesn’t have anything outside of that.
Being on Cartoon Network, alongside shows like Steven Universe, I do think that we felt like we could do more than we could have back then. Rebecca [Sugar, who created Steven Universe,] really did so much of the heavy lifting that I feel weird even talking about that too much.
It was something that is important. When it showed up in the storyboards, it made sense. You just have to do it enough so that it’s not a new story, and then it will just be something that happens. Hopefully it’s not important in that way. Hopefully it’s not something that we even need to comment on, even.