...those who are considered worthy to attain to that age and to the resurrection from the dead neither marry nor are given in marriage, for they cannot die anymore, because they are equal to angels and are sons of God, being sons of the resurrection.
Luke 20:35-36

It is not the immortality itself that negates the need for marriage, because marriage was instituted before death came into the world. It must be immortality plus something else. Perhaps it is the great number of believers who will be resurrected or a change in humanity's mission that no longer requires procreation or the exclusive partnership of man and woman.