Marsha and Tails first met in "Adventure Time 6: Creepy Fears." Where My Little Pony and Sonic and his friends from Sonic X team up with Adventure Time to take out their evil counterparts from creepypasta fan fictions. When Tails first meets Marsha, he imeditly falls in love with her and her beauty, since he calls her "beautiful" in the movie. He admits that he has a crush on Marsha is The Ultimate Enemy and actually faints when she kisses him on the cheek at the end of the episode.
Marsha as well has feelings for Tails. In "Adventure Time 6: Creepy Fears", she calls him "cute" and threatens Sonic.EXE that "she'll rip him to pieces if he hurts one of the three hairs on Tails' head". At the end of that same movie, she is seen painting a picture of Tails, showing her affection. In "The Ultimate Enemy", she is taken to Tails' garage and even finishes the teleporter for Tails when he is busy arguing with Knuckles. In the song "It'll Be Okay," Marsha sings about her love for Tails, showing that she is no longer in love with Gumball and is deeply in love with Tails, being one on the main reasons why she left Adventure Time.
That couple is pretty cool. Cameron is here 17:33, August 25, 2013 (UTC)
Tails is 8 years old, duh (Well, according to your timeline about how old Tails is). It's awkward to see Tails kissing Marsha, IMO. "You've got this, Sonic!" —Shadow