In Adventure Time, there are two possibilities:
A) The screenwriters couldn't pick BMO's pronouns
Or
B) We have a genderfluid character
Add that to Marceline and Princess Bubblegum dating, and you have the first pro-LGBTQ+ Cartoon Network show directed at kids (specific but ok)
*goes to rub it in my sister's face cuz unlike me, she doesn't like Adventure Time even though it's awesome and was my childhood along with WWE and Ninjago* (ye I have a weird childhood, fucking deal with it)