That said, If my son or daughter were to invite me to such an affair, I would attend out of my love for them - not to condone the "union".
If I was still a part of the Catholic Church - born into it but never "got it" and wandered from age 14 to age 50 before hearing the message of Christ's Love and Sacrifice and turned myself over to Him as my Savior - I couldn't imagine not attending a traditional marriage of someone I loved because they were divorced. I can see not condoning it, but have a bit of trouble with shunning the relationship, even if only for the ceremony vs. everyday life.
My attitude is that God's Laws trump our laws and He defined marriage and we can pervert the concept at the human level, but we cannot pervert what He has wrought and decreed. Ergo, there is really no such thing as gay/homosexual marriage - it all becomes a human-defined construct and with no real meaning.
At any rate, I appreciate the response and the Catholic point of view - while I don't have much use for religions, I understand that even the parts I disagree with have some potential value as far as getting the Good News of the Gospels out and most of the souls saved since His sacrifice have been under the aegis of religion.
Sometimes we have to disappoint our children and even do things that, on the surface, might appear unloving. This is known from experience.
I would not attend...out of love for them.