I agree with this article. Here's an portion of it:
http://www.thegospelcoalition.org/article/should-i-attend-a-same-sex-wedding
There are two competing tensions to reconcile in answering this question:
The fact that you love and care for the person getting "married" means the answer should be "yes.” It is difficult to see how not going to the ceremony demonstrates love and care for the person.
The fact that this is not really a marriage at all means the answer should be “no.” It is difficult to see how going to the ceremony communicates anything other than your approval—so if you don’t approve but go anyway you act dishonestly or with hypocrisy.
My personal position is that answer #2 above outweighs answer #1, so I would not go. Sometimes the most loving thing to do is not the thing that on the surface looks most loving. I don’t want to do something in order to make someone else feel more positive about me if by doing it I am in effect encouraging them to do something harmful. Going to a same-sex wedding is not the most loving thing to do because I don’t want to encourage my friends in actions that run contrary to God’s command.
http://www.thegospelcoalition.org/article/should-i-attend-a-same-sex-wedding