Six in the third place means:
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.