Therein lies the problem. God has not changed. He has not issued a repeal of the 6th commandment. Why are we changing teaching? Considering Jesus was less tolerant of divorce than the pharisees, it is particularly foolhardy to think the Church's constant teaching has been in error for 2 millenia and we are only now getting it right. To assert that the teaching has been wrong for that long asserts that God has loved us so little and been so merciless that He has allowed us to continue in error for so long and that the church has no particular charism for the truth.