It is critical to the plot.

It is critical to the plot and I am tired that people think that it's unnecessary when it basically is massive character development.

Actually for two reasons. Misato was just teasing Shinji and doesn't know how to be a mother, only a woman. Kaji and Misato aren't a casual fling, their relationship is placed on hold because of the Angels but they decided that the world was ending and it didn't matter anymore.

PS: Also Evangelion was Seinen. It was an adult show for adults that where teens when Super Robot Mecha was popular.