The song was sung by Marcelina (voiced by Olivia Olson) about her feelings for the Bubblegum Princes, in the episode "Marcy & Hunson" - the seventh episode of the tenth season of the American animated television series Adventure Time.
Wikipedia quotes the songs writer Babeo Baggins about the background for this song:
"So, I wanted to give you guys a bit of a backstory on the song I wrote called "Slow Dance" ... I wrote [the song] about this girl I had a big crush on, and she liked me too, but ... she had a boyfriend and I wasn’t a boy... (to her). I identify as genderfluid, and I feel very he/him most of the time. I was so heartbroken because I knew I wasn’t a "boy" in her eyes but I thought I could be, if she let me. Hence, "I know all the other boys are tough and smooth". It’s okay tho [sic], she’s a very lovely girl and I wrote a really good song out of it. I’m happy you all like it so much. Just wanted you all to know the root of the song."