Six in the third place means

The marrying maiden as a slave.

She marries as a concubine.

A girl who is in a lowly position and finds no husband may, in some circumstances, still win shelter as a concubine.

This pictures the situation of a person who longs too much for joys that cannot be obtained in the usual way. He enters upon a situation not altogether compatible with self-esteem. Neither judgment nor warning is added to this line; it merely lays bare the actual situation, so that everyone may draw a lesson from it.