Marceline is the only character who is on a First Name Basis with Bubblegum, not even her close friends calling her Bonnibel, and also calls her cute nicknames like "Bonnie" and "Bub-Bubs".
In "What Was Missing", Marceline sings a song (which also contains the line "I'm gonna drink the red / From your pretty pink face") that is about how much it hurts her that Bubblegum is hostile to her, especially when she has no idea why she's treating her that way, and for some reason she doesn't understand she still wants to "make up" with her despite the pain and to fix whatever they have. She was on the verge of revealing something incredibly intimate (powerful enough to almost open the door) until she lost her nerve at the very end.
After Bubblegum's attempt at a song fails, Marceline taunts her that "she's not as perfect as she thought she was" and has no right to judge her, to which Bubblegum angrily replies that she "never said [Marceline] had to be perfect" (the original storyboard has her add "I just want you to not be so much of a screw up all the time!"). In response Marceline spits at Bubblegum, Bubblegum storms off in shocked anger, and Marceline actually starts crying (for one of only three times in the entire series) and also leaves.
At the end of the episode, Bubblegum's stolen treasure is shown to be a shirt Marceline gave her which she wears as pajamas (for comparison, Finn's treasure was a wad of Bubblegum's hair when he was still in full Precocious Crush mode towards her), a revelation which causes Marceline to blush.
A line of dialogue in "I Remember You" suggests that, at the very least, Marceline is not heterosexual.
Ice King: I'll get all the princesses and you can have... whatever it is you're into.
When the Ice King was singing a song about Princess Bubblegum, Marceline is briefly shown looking very uncomfortable...but singing about the other princesses didn't seem to cause any reaction.
In "All the Little People" when Finn empties the bag of tiny replicas, for a brief moment, Little Bubblegum is in Little Marceline's arms, until Finn separates them. Later, they can be seen walking off together.
"Sky Witch". Holy crap "Sky Witch". The episode literally begins with Bubblegum getting out of bed and sniffing the shirt Marceline gave her in a sensual fashion. Right after this we get a shot of a photo showing Bubblegum and Marceline together in Bubblegum's closet. At the end of the episode the Sky Witch says the shirt Marceline gave Bubblegum has more sentimental value to the two of them than Hambo (for comparison, Hambo is pretty much the only thing that Marceline has left of Simon before he went insane and became Ice King).
This page◊ is... very open to interpretation, even in context.
Marceline: I can also turn into a tentacle monster. [does so] Bubblegum: How does that help our situation? Marceline: How does it hurt our situation, though?
Issue 3 has several moments, such as Bubblegum's worry over a scratch on Marceline's cheek even though everybody at the time is hurt except Bubblegum and Finn. Then there is this line after Bubblegum gives Marceline her crown so that Marcy can fight the Lich.
Bubblegum: Marceline! Remember you promised to give me back my crown in one piece! Bubblegum: ... Bubblegum: (whisper and worried look) Don't get hurt.
In Marceline and the Scream Queens Issue 5, when Princess Bubblegum meets Marceline's dad◊, it plays out a lot like that dreaded moment when you have to introduce your girlfriend to your parent(s). His "I've heard so much about you!" suggests he realizes Marceline and Bubblegum are very close, and Marceline's response would be a bit of an overreaction if she weren't dating (or hadn't dated) Bubblegum.
Later in the same issue, Bubblegum compliments Marceline's pants. Then when they get torn (thus showing more of Marceline's legs) Bubblegum comments that she likes them better as shorts.
In issue 24, at one point Marceline falls in monster form on top of Bubblegum. Then when she's getting up it... looks pretty suggestive.
Issue 25 is pretty much an entire issue of PB/Marceline teasing. Their close relationship? Goes back centuries, even before PB made the candy people. The gem Bubblegum was shown to have in "The Vault"? It's part of a pair of "friendship amulets", the other of which she gave to Marceline as a present. The robot that Bubblegum built? Programmed to not hurt Marceline, and only Marceline. The reason for their falling out? Bubblegum made the Candy Kingdom causing her to spend less time with Marceline, and she become jealous, feeling that Bubblegum was abandoning her. Furious and upset, Marceline gives an impassioned speech which she thinks Bubblegum will never hear (but she is listening, and by the end she's crying) which is... certainly something.
Marceline: You there? ...No? Well, good. Cause I got some things to say to you and I don't want you to hear them. I'm complicated, alright? You should know that by now. Okay, so I'm not stupid. I know our lives are moving us in different directions, okay? I get that. I get that nothing lasts forever, believe me. But that doesn't mean I have to like it. It's just - the way you act like nothing's wrong, it makes me SO mad. It makes me feel like I'm the only one who remembers how our friendship USED to be, who sees how it's changing. It's feels like a betrayal. It feels lousy, Bonnie. And familiar. This isn't my first rodeo. And I know that you've got your hands full with you new kingdom, okay? I get that you're busy, and I want you to be busy. You're doing something so ambitious, so crazy... I know you can do it. You're gonna be the one who changes the world, Bonnie. I've seen it in you. But there's gotta be a balance there, you know? A way where we can stay friends, a way where I don't feel so hurt all the time. And it's not all you - I know I've been major cheesed because of this and that's not fun for anyone ...I'll be a better friend, too. [...] You've been my best friend for so long and we're not gonna lose that. We'll figure this out, Bubblegum. Huh. "Princess Bubblegum." You know, it actually DOES sound kind of cool. My friend, the princess. I think I could get used to that.
Edging slightly closer to Word of Gay, a video on the show's Mathematical promo channel on YouTube definitely reinforced (though not confirmed) the "Bubbline" phenomenon in the "What was Missing" episode. While the video primarily played the idea for mild titillation, it demonstrates there is an appreciation for the pairing "in-house", if nothing else. It was later pulled down and disavowed by the show's executive producer, and it should be noted that the people behind the Mathematical channel weren't part of the actual staff.
Considering Gender Flipping, it can be assumed that Prince Gumball and Marshall Lee have a similar past.
One page◊ from the◊ Fionna and Cake comics has Fionna complaining that she wasn't invited to Marshall Lee's and Prince Gumball's "Boy's Night". They decide they can't keep secrets from her anymore and look like they're about to hold hands. Fionna and Cake's reactions prove that the implications were all intentional.
It's a bit brief, but in Fionna and Cake issue 5, Fionna passes by Marshall leaving the Candy Palace, carrying a very sugary parasol to protect from the sun. He says he just woke and from a nap and asks Fionna to tell Gumball he said bye. Fionna pauses to think "Marshall and Gumball are pretty cute pals." Pals, huh?
In the F&C comics is a four page comic called Sweaters, where each character had their sweaters stolen from them. At the end Gumball revealed that not only had he lost his sweater, but the sweaters he had made for them, including Marshall Lee. His is maroon with bats going across the middle.
Lemongrab & Lemongrab 2
Lemongrab and Lemongrab 2 ooze this trope out of their lemony pores from their first meeting. The poking, the Eskimo kisses, the cuddling... and Lemongrab has no problem with walking up to the nude guy and poking him repeatedly. PB explicitly states that she made him for Lemongrab to be with.
"Stop by any TIIIME!!!" "But call first!" (a very common thing for recently married couples to say to in-laws in real life), they say to Princess Bubblegum and the others after exchanging interesting smiles of mutual bliss.
The Adventure Time Facebook page described their relationship: "They keep each other company in lemony bliss." They sound like a married couple!
At the end of "You Made Me," for just a brief moment, Lemongrab is shown making this face◊ at Lemongrab 2.
EVERYTHING about the Lemongrabs in "All Your Fault". The way they speak to each other and their children, the way they bicker about what to name their kids, the promo art; Adventure Time basically has a gay couple who live together and have a bunch of children.
There is a Freeze-Frame Bonus screenshot of Lemongrab's brain, in which his thoughts are visible. One of them refers to a "beautiful lemon man."
Thomas Herpich, one of the artists on Adventure Time who wrote "You Made Me" and "All Your Fault," was politely asked to draw a picture of the Lemongrabs for a fan at the 2012 Brooklyn Comics & Graphics Festival. Having been given no other instructions on this vague request, Herpich responded by drawing a picture of the Lemongrabs hugging and smiling... and butt naked◊.
The two are the stars of an Adam and Eve Plot. Lemongrab, like Adam, starts off totally alone and the first of his kind. Princess Bubblegum (whom LG calls "Glob", or God in Adventure Time speak) makes another Lemongrab so he doesn't get lonely, and eventually they have children together and start a whole new civilization.
Their genderbent counterparts in the Fionna and Cake comic books acknowledge all of this lemony Ho Yay. Kate Leth, who created the genderbent "Lady Lemongrabs" for the backup story "Sour Candy", drew this self-proclaimed "homoerotic" illustration one day before the release of issue 3, Sour Candy's debut. The comic itself also contains a bit of Ho Yay, too, including vague implications of an anniversary party, hand-holding, and face-nuzzling.
In the first "Adventure Time Annual" issue, there's a 6-page comic called "Lemon Sea." In the story, the Lemongrabs basically go on a date. They leave their kids at home and go to their own private beach for a picnic.
"Another Five Short Graybles" both ships and sinks the two by having the two refer to each other as brother but also refer to each other as a dad to their fake doll son.
"Too Old" has Lemongrab act like an abusive husband and father to the other Lemongrab and their children, then eats him, causing pretty much everyone to jump ship at that point.
In "Too Young", Lemongrab (unintentionally) sucks on Peppermint Butler's head, and afterwards smacks his lips a couple of times and says "Mmmm!" to himself pleasantly. Neither of them seemed to mind at the moment.
Finn: There's a big sleepy lava man in our front yard! And he is so hot! Jake: Mm-hmm... (nudges Finn) Finn: No—no wait, I take it back—I don't mean like... sexy hot... Jake: No... you do mean sexy hot!
From "It Came From The Nightosphere":
Marceline: You can't hurt my dad, you're like an ant to him. Finn: Well this ant's about to get in his pants! Marceline: (raises eyebrow)What.
In "Hug Wolf", Finn hints that he has "repressed emotional feelings" for Cinnamon Bun.
Finn and Jake themselves have some of this. They're brothers, so it's expected they'd be close, but some things bring it to strange levels.
From "Gut Grinder":
Finn: You can't be stealing gold in your sleep. I watch you while you sleep. Jake: What?!? Yo, Finn, that is creepy, man. Finn: I can't help it! I take pictures!
From "The Silent King":
Jake: Hey, Finn's gonna need a queen. (beat) I'll do it.
From "The Eyes":
Finn: If you weren't so darn handsome, animals wouldn't always be staring at you! Jake: You're just mad at me for being... (smiles) handsome?
Ice King's obsession with being Finn and Jake's "friend" is half pathetic, and half thoroughly creepy. "Still" is disturbing even by his standards, what with paralysing the two of them and forcing them to perform bizarre activities with him, including cooking them an omelette that he's stuck his foot in (so that "part of [him] will be inside [the two of them]").
The comics have Ice King claim that his and Finn's destinies are cosmically intertwined.
Finn and the Ice King cuddle in "Play Date". Finn is okay with it.
On the subject of "Fionna and Cake", Fionna spends most of the episode on a date with Gumball, complete with Gumball saying she "looks beautiful in the snow" and singing her a romantic song. Except it was Ice Queen in disguise. Fionna went on a date with Ice Queen.
The song in question is dripping with Foe Yay, with such lines as "If I said you're a beautiful girl would it upset you?", "Your fist has touched my heart", and "I'll never let anything in this world keep us apart". It makes a lot more sense when you know the twist.
In the Fionna and Cake comics, Ice Queen admits that she doesn't hate Fionna, it's just that Fionna is always in her way. During the climax, Ice Queen takes the salt crystal out of Flame Prince's body, and absorbs it into her own. She says (to herself) she thought she needed the love of a prince, then the baby fire lions, and then she says the crystal could hold more warmth than both of them combined. As she's melting, she says "There's so much strange magic in this world... be careful with it, Fionna."
The marauders from "Memories of Boom-Boom Mountain" like to wrestle. And then they like to pet each other. Pet each other raw.
From the same episode:
Mountain: Those men and their disgusting fantastic bodies! Jake: This guy has problems.
In "Death in Bloom", Death kisses Jake on the lips to restore his memories, with a Foot Pop from Jake.
"Kiss of death, baby."
The very first gladiator in "Morituri Te Salutamus" (who are later revealed to have been forced to kill their partner gladiator by the Fight King) mournfully yells "Hyperion! My love!", and the other gladiators are all either crying or expressing similar dismay. At the end, after Finn kills the Fight King, all the gladiators are paired off, holding hands, and disappear together. It was later confirmed that all of the gladiator pairs were in fact same-sex couples.
In "The Hard Easy", Finn kisses a frog, who turns out to be a buff prince. Complete with bright, sparkly magical boy transformation. Plus, he calls Jake "sweetles" at one point.
Prismo has a lot of this towards Jake. It's mostly one-sided since Jake's too oblivious to notice it at all.
Jake: I gotta get that guy a girlfriend.
"Wizard Battle" has Huntress Wizard competing for Princess Bubblegum's kiss, and was seen cheering after hearing about the prize (Word of God insists she was more interested in testing her magic powers against the other competing wizards).
Also in "Wizard Battle", Abracadaniel gives off a gay vibe since everything about him is soft and delicate, his speech included, he uses rainbow magic, is outright repulsed at the idea of kissing Bubblegum, and is shaped like a certain body part. Also when he "wins", he states that he deserves a kiss (not that he wants it, wording is important).
Whatever BMO did here◊ in "Jake the Dad". While BMO is mostly in the Ambiguous Gender camp, other characters and BMO themself often treat them as male.
Whole "Davey" episode. With Ice Cream person angrily asking Finn out for a dinner, then telling him that he made him one. And with Davey (Finn in disguise) building a shack together with another Candy Person, and then hanging around together in it.
Jake eats Meat Man's meat in his sandwich. That night he has a dream about Meat Man, the following morning this exchange happens,
Jake: (staring out the window with a cup of coffee and a blanket wrapped round his shoulders) I had a dream about Meat Man... I think I'm going to stop eating Meat Man.
Bubblegum makes out with her left hand in "A Glitch Is A Glitch", which is later revealed to be sentient and female.
"BMO Lost" has BMO and Bubble, who try to adopt a baby together, and ends with Bubble proposing to BMO (granted, he admitted he didn't know what "married" means). BMO said yes.
Towards the end of "The Suitor", Peppermint Butler has a dark magic spell cast on Braco to make him a "love magnet". It goes horribly right.
Braco: Did it work? Am I a love magnet?
Peppermint Butler: Well, I want to have your babies.
Later at the end of the episode, when Bubblegum tells Peppermint Butler that she sent Braco away, he angrily responds with "You should've given him to me!".
Tiffany has a rather unhealthy obsession with Jake. In "One Last Job" Tiffany flat out yells "I LOVE YOU JAKE!".
In "The Silent King", Gummy the goblin seems all too eager to be spanked in the buttocks by Finn, and sits on the lap of Whisper Dan, while thinking that he's Finn.
In "Slow Love", Snorlock keeps wishing that Finn and Jake were girls so he could date them.