You wish they would not, but you accept them as they are.
There are too few close people in a lifetime to pull a Cotton Mathers.
That said, Gay marriage is stupid play-house, most people feel that way, but giving someone close one special day is no big deal.
True, that does not mean that we cannot place conditions on the people we love. They receive our love unconditionally, but we don't accept everything they do. It is not the same thing He can still love his cousin and not attend.
In fact, real love requires that you do that right thing for the person, even at your own expense. In this situation, not attending is doing the right thing. Attending is helping the cousin feel better about his very wrong decision -- a decision that may cost him his soul if he does not one day repent and leave the man.