As a child, Rebecca Sugar always loved those “animated movies that ended in a beautiful wedding between characters that only met a few days earlier”. Even if, as a queer, non-binary child, she didn’t always feel reflected by the strangling gender norms of Disney films.

The 32-year-old animator is now enjoying – and adding to – a very different cartoon landscape. This year, viewers of the children’s show Arthur saw Mr Ratburn marry an aardvark (and chocolatier) named Patrick. In 2017, the Disney animation Doc McStuffins aired an episode featuring an interracial lesbian couple. And, last year, Sugar’s own series, Steven Universe, became the first kids’ show to feature a lesbian wedding.

For self-professed geeks in the LGBT community, Sugar is something of a gay-eity. In 2011, she got her break as a writer and storyboard artist on the animated series Adventure Time. She earned six Emmy nominations and a slot on Forbes magazine’s “30 under 30 in entertainment” list for her contribution to many of the best episodes. There, she laid the groundwork for the cult character Marceline, a bisexual vampire queen and one half of the queer cartoon couple dubbed “Bubbline”. In the show’s finale, she and her girlfriend, Princess Bubblegum, sealed their largely offscreen romance with a long-awaited onscreen kiss.

All hail Bubbline! ... Marceline and Princess Bubblegum in Adventure Time. Photograph: Cartoon Network

Sugar left to continue her work moving LGBT stories from the margins into the mainstream with Steven Universe. In doing so, she became the first woman to independently create a series for Cartoon Network. It is an all-ages sci-fi romp with lots of heart. The titular character is sworn to protect humanity from intergalactic threats, alongside a ragtag gang of ageless alien warriors called Crystal Gems. They project female holographic, humanoid forms from magical gemstones embedded in their bodies, which are akin to brains. This may sound complex, but the show is renowned for its simple approach to often inaccessible themes for children, such as consent, grief, anxiety and abusive relationships.

Sugar uses the show to process difficult parts of her personal life. When she was 22, she and her partner (the show’s supervising director, Ian Jones-Quartey, who is black) were the victims of a racist assault. This is addressed in the fan-favourite song Stronger Than You, which is performed by the character Garnet, voiced by the British R&B singer Estelle.

“There are ways I can talk about my relationship with Ian and the obstacles we face, my own experience with antisemitism, my experience coming out as bisexual as a teenager,” she says. “The layer of fantasy in the show allows me to have fun with truths that are not fun.”

Garnet is a queer Trojan horse of sorts. We are introduced to her as a singular character, but it is later revealed that she is a fusion of two Gems called Ruby and Sapphire, who choose to remain fused out of love for each other. When they were married, Ruby, the more masculine of the two, was depicted in a bridal gown, while femme-Gem Sapphire sported a tux – a nightmare for countries that had skirted the depiction of a same-sex relationship by coding Ruby as male.

‘I can really make characters behave and act and exist outside of the rules that always felt so alienating to me’ ... Rebecca Sugar. Photograph: Jeremy Freeman/Cartoon Network

The recent push for diversity in media has seen a rise in “retcon” (“retroactive continuity”) – the addition of new information that alters previously described events. For instance, JK Rowling belatedly revealed that Dumbledore and Grindelwald from the Harry Potter novels had been lovers, a move that was deemed a bandwagon-jumping afterthought. But Ruby and Sapphire’s relationship has been key to Steven Universe since the start. “We wanted you to understand that you already knew Ruby and Sapphire, because you already knew Garnet,” says Sugar. “We tried to bake that into the premise – you already love and care about the relationship between Ruby and Sapphire, because you already love and care about Garnet.”

The wedding decision was not taken lightly. Sugar was aware that the episode, as well as her decision to come out as bisexual, could lead to funding being pulled or the show being dropped altogether.

Her fears were hardly unfounded: Arthur’s first attempt at LGBT inclusion was shot down in 2005, when its spinoff show, Postcards from Buster, was pulled from PBS after taking flak for featuring a lesbian couple. The anime show Sailor Moon featured a lesbian couple, whom US TV networks opted to portray as cousins. More recently, a Steven Universe episode featuring a “fusion” scene with a lesbian subtext was censored in Britain. The same episode featured heterosexual dancing and kissing. “I decided I would rather do this and face that reality than not do this and have a longer running,” she says.

“It was a matter of breaking down the false assumption that same-sex romance is somehow inherently more adult, which it’s not. We needed to show, over and over again, that there was nothing less wholesome about these two characters together than any other animated wedding ever aired on Cartoon Network. And so we worked hard to make these characters as cute and lovable as possible, so there’s just no way to deny that they should be married.”

‘His emotional intelligence is his greatest asset in battle’ ... Stephen Universe: The Movie. Photograph: Cartoon Network

Steven Universe, which has as many adult fans as child fans, isn’t just deemed radical and groundbreaking for its same-sex relationships. One episode nods to polyamory, with a character who consists of six romantically involved Gems called Fluorite. Its exploration of gender is just as celebrated: Steven, a male, and his friend Connie, a female, fuse to create the gender-neutral Steveonnie, who uses “they/them” pronouns. Most notably, Sugar has said that the Gems are non-binary women.

“I was writing these characters before I was familiar with a lot of the terminology,” she says. “But it was such a fun way to express myself – particularly in the way that people think they are women, but I know that they are actually not. It is so much the feeling I’ve had navigating my life. I can really make them act and exist outside of the rules that always felt so alienating to me.”

This year, Queer Eye’s Jonathan Van Ness and the musician Sam Smith have come out as non-binary, which has been met with equal amounts of support and derision. “But what’s exciting now is that we have ways of finding each other,” Sugar says. “Even if the confusion exists, we are not confused.”

Steven himself (based on Sugar’s younger brother) is a direct challenge to the archetypal male hero in most cartoons. His emotional intelligence is his greatest asset in battle. He cries, he uses a pink shield and he is often depicted in a dress; in the episode Sadie’s Song, he dons makeup, a crop top, a skirt and heels.

Cultural shifts such as this have led critics to suggest we are in a golden age of western animation – shows such as BoJack Horseman, Rick and Morty and Big Mouth also deal artfully with the unsaid. Sugar is held up as a beacon of progress, but she is keenly aware of just how rare a gem she is.

“Often, when there are these moments, if you really look at it, it may be coming from two or three people that make several things,” she says. “You think: ‘Wow, this was an era of really innovative storytelling!’ but it’s just a few individuals fighting for something they really believe in.”

Steven Universe: The Movie is on Cartoon Network on 1 October at 8pm