Heinberg clearly wanted the Iron Lad/Vision/Cassie triangle to parallel the famous Vision/Wonder Man/Scarlet Witch triangle, where you have two men, one of them an android with his brain patterns based on the other man, in love with the same woman.
But his story also required that Iron Lad would leave at the end of the first six issues, so he had to rush that.
Probably the writers in between Young Avengers and Children's Crusade should have done more to flesh out her feelings for Nate, because in Children's Crusade it seemed to be kind of out of the blue that she still had feelings for Nate.