Melody isn't ace, and Andrea is straight, as is Seth. Melody is closeted, however, and uses Seth as proof of attraction to men. He refuses to reciprocate, especially as he loves Andrea and cares for Dylan, and simply helps Melody figure herself out. After a long time, Melody comes out to him then Dylan as bi, and accepts that she has sexual desires, and that they're directed to both men and women, and becomes involved with Dylan. It's a struggle at first, but Dylan nurtures her into it.