One factor to consider is that we cannot know your parents’ hearts and why they chose to attend. When my son , a young adult who chose not to be confirmed, is acting in a manner inconsistent with God’s teaching …sinfully…I express my disapproval unequivocally but do not harp at him or ostracize him. I consider it my life long duty to influence his faith and morals and I cannot if we replace our relationship with either anger or distance.
My son is straight but If I had a gay child entering into a gay marriage, I would tell my child privately my position on the matter, grounded in Catholicism. I would acknowledge his free will. I would avoid gossiping or complaining about it and would attend the wedding. I would dress for the social expectations of the event and be pleasant. I would ask not to be singled out and not to participate in the ceremony as parents sometimes do. I would be there for one purpose…to maintain my close bond with my son. As in, sure, my son could escort me in and I would sit and enjoy the music and so on, but I would not want to be presented flowers or light candles during the ceremony, for example, as mother’s sometimes are honored. I would gladly wear flowers from my son of given to me privately. The difference is not participating in a marriage ceremony that I consider invalid.
I have been to many weddings which I thought were ill advised. I have had relatives who were too immature to marry, and friends who picked poor matches. My personal free will had zero bearing on the outcome. These marriages all ended in divorce, some annulled (not all were Catholic people,) and because I did not anger or ostracize the individuals, I was able to help them do better later.