Leaving aside the “gay” question, this is a very important event in your cousin’s life, and a day on which he expects to celebrate the happiness he’s found with his partner. If it were, say a ceremony of a different religion - say, Wiccan, for example - would you still be asking this question? Or if it were simply a non-religious ceremony with a justice of the peace officiating? If it were me, I’d want the people I love to come and share in my and my partner’s happiness. It’s just my opinion, but I think going will send a powerful message to your cousin that, despite your disagreements, you love him, you always will love him, and you want him to be happy.
Or, perhaps another way to put this question is to turn the tables a bit. If this were your (Catholic) wedding, and your cousin refused to attend because he’s one of those Protestants who think the Catholic Church is the “Whore of Babylon,” how would you feel? Answer that question, and you’ll be very close to understanding how your cousin will feel if you refuse to attend his wedding.