A child is one sure sign of the oneness of a couple as they both contributed to forming that child and once born is indivisible and inseparable for sharing between husband and wife.
“And Adam said, This is now bone of my bones, and flesh of my flesh: she shall be called Woman, because she was taken out of Man.” – Genesis 2: 23
“Wherefore they are no more twain, but one flesh. What therefore God hath joined together, let not man put asunder.” – Matthew 19: 6
“One Lord, one faith, one baptism,” – Ephesians 4: 5