Greg goes into great detail on his reply. My own sense is that nearly any situation would be unique. The number of such unions will always be relatively small, especially since SSA people are not forcing married heterosexual couples into divorce in order to marry someone of the same sex.
If a close family member of mine (daughter or sibling) were gay or lesbian, I would probably avoid scandal in the domestic Church and attend. My attendance signifies my approval of same-sex unions to the degree that my American citizenship and tax payments indicate I support adventurism in southwest Asia, Guantanamo not-closing, or welfare CEO’s.
I might not take an official role in the proceedings. But then again, at my daughter’s wedding, I’ve already mentioned to her I don’t believe in the father “giving away” the bride. She will be an adult woman–she gives her own consent when she marries some guy.
I think that if one is going to take a moral stand by non-involvement, one has to be thorough in the degree of immorality that has entered into the world. If Catholics, or anyone else, was so focused on not “supporting gay marriage” that they only boycotted lesbians and gays, that person might have genuinely moral heft behind their view. But they would also be prejudiced against SSA persons, to the degree they did not boycott people involved with other grave sins against common decency.
I could foresee being asked to provide music for a same-sex religious service. I will admit I have played music for Catholics marrying outside of the Church. Those instances are discerned on a case-by-case basis. I have also declined to be involved as a minister, and have just attended non-Catholic ceremonies.
Some people might think that by attending that my position as an “official” Catholic staff person compromises the Church and our teachings on various things. I prefer to look at it in another way.
Most people realize that the Catholic Church does not move in lockstep. My presence says nothing about Catholic approval or disapproval of something, and most adults know it. But my presence is also a Catholic one in the sense that I’m always open (or trying to be) to share my faith and encourage others to explore the Catholic faith. I have had many sparkling conversations with non-Catholics, inactive Christians, and even non-believers in the most interesting of settings in which a deacon or priest would perhaps decline to be caught dead.
I try to be on the lookout for ways the Holy Spirit inspires me to engage with people. So while I’m not representing the institution in a mistaken gesture of approval, I am going as a sinner among sinners. Or, if you prefer, as a disciple among potential seekers.
It’s complicated. And there are no easy answers.