You love this cousin, and he's your family. If he were straight and marrying a woman for money, or to get her a green card, or to get even with his ex, or for some other highly suspect reason, you'd still go, wouldn't you? And hope that he will be happy, even though you may not "approve"?
He's found someone he loves and wants to commit himself to. Even though you are firmly against the law being changed to allow gays to marry, that doesn't mean that his happiness is any less important, or less to be celebrated. Gay couples (as well as straight ones) know perfectly well that not everyone who attends their wedding is going to be jumping with joy; but they hope that such people will be "won over" by witnessing their happiness.
I would go, and have a wonderful time.
Except for the green card situation, which is illegal, these examples are comparing apples and oranges. This is not about not liking the partner, or being comfortable, or going along to get along. Suspecting someone is marrying for money is not the same thing has knowing he is marrying another man.
I can't believe this thread. FReepers are indistiguishable from leftists here.