The reason behind the debate is because the Code of Canon Law states: “Only those marriages are valid which are contracted before the local [bishop], pastor or a priest or deacon delegated by either of them who assist, and before two witnesses” (1108 §1).Canon law does not prohibit Catholics from attending invalid weddings, Father Pacwa [EWTN] explained, but he said that Catholics must discern carefully. “Every situation will call upon our reserves of prayer, discernment and evaluation,” he said. “And a good confession before making any decision is always a good idea.”According to him, maintaining peace within families is very important, but there is also the question of motive: “Are they trying to put me in an uncomfortable position to prove their point? Or do they simply not see that there is an issue at stake?”
I guess the question then would be 'what constitutes a witness'?
To see the complete turn around on this Catholic teaching to what is now allowed visit: http://catholicdoors.com/misc/marriage/canonlaw.htm