I would not go because I would be uncomfortable, but would send a nice gift (= expensive, from Tiff's or somewhere they would like)to show affection. Your cousin is not going to change his sexual orientation because you made it clear to him that you disapprove, so sending little messages is pointless and nonproductive.
Hurtful, in fact. Especially since you are close to him.
Maybe you should consider attending the reception. Show a little love to someone you love...just like he did for you.
However, going to some sacreligious "ceremony" should not be part of the itinerary.
That said, I would not be a very good guest at something like this....I just can't watch two dudes making out without becoming physically ill, so I would probably be outside smoking or something for the most part.