I kinda think I would go, and here's why:
"Family" ain't the rest of the world. They are not the media or the politicians or anybody else.
They are the ones who stand by you no matter how bad you screwed up.
And it sounds like you and this fellow are really close.
If you chose to go, you can always make it clear to him before that you're going NOT TO RECOGNIZE GAY MARRIAGE but SO THAT HE CAN REMEMBER YOU BEING THERE ON A SPECIAL DAY.
JMHO.
djf has given you the best advice IMHO. You can pick your friends, but you can't pick your family.
I might draw the line and NOT go... IF it were being done in a church. Personally, I could stomach "civil union"... but, not a church sanctioned ceremony.
This is an untenable position. What is "special" about a sexually immoral public ceremony? You cannot have it both ways.