Marmalade Boy

Plot Summary (Courtesy of The Anime Web Guide):

Who do I love? I love...Arimi! Wait, that's not right! I'm so confused!

Miki is a perfectly normal high school girl. One day, her parents tell her they've decided to swap spouses with another couple they've recently met. The idea horrifies Miki, but she discovers that the other family has a boy her age (named Yuu), and she falls for him. She relents, and the families move in together. Suddenly, Yuu's old girlfriend Arimi shows up. Then Miki discovers that the boy she had a crush on in junior high (Ginta) is in love with her! And he starts hanging out with Arimi! It gets even more complicated from there.

Marmalade Boy links (taken from the Anime Web Turnpike):