I don't think I'm going to support my cousin's upcoming marriage.
Why wouldn't you support your cousin's impending marriage?
I do not think my cousin is happy about the wedding.
It is bad for you to be unsupportive of a marriage in your family. Maybe you can talk to your cousin privately and try to better understand him.
Talking to my cousin is pointless. When he gets a divorce it'll prove that I was right about this.
That doesn't seem very supportive, and if he finds out you're thinking like this then you may damage your relationship with him. Instead of making assumptions about your cousin's marriage, try to have a talk with him and see if there's anything you can do to help.