No you are not doing the right thing, unless you just want to hurt his feelings very much. In that case you would be doing the right thing.
However, if you can remember for a moment that you love your son, and that you really don't know everything, then you can also remember that you supported your son when he was only 3 years old and exploring the world, and you supported your son when he was starting school, and you supported your son when he was in high school - and the whole time he was just trying his best to figure out how to put meaning into his life.
But now, you suddenly withdraw your support, because you think this is a mistake, but that all the millions of other people getting married ARE NOT making a mistake.
I think marriage is a mistake for everybody, but for the love of your son - go to his wedding since you were invited! You could permanently damage your relationship if you don't show up. Basically, you are asking your son to choose between his parents and a person he loves. You are going to lose that contest - and regret it.
Just show up and have a good time.