Almost everyone in my family knows I'm about to marry a woman on May 1st. They've all met her. She's really great and I am getting support from most people, (Aside from the usual "its too fast" blahblahblah). However, I have not gotten the change to tell my Grandpa I'm Gay, let alone getting married in two months. Its pretty nerve wracking!!!
My grandpa was raised roman catholic and was in the military for several years. He's ultra conservative and has never been exposed to things like gay people/gay marriages/even interracial marriages.
I don't know how to break the news that not only am I gay, I'm getting married in two months. He's a very opinionated, sometimes angry guy. (My Aunt told him she was getting married to someone he didn't like and he punched a hole in the wall.)
There's a definite possibility I might lose him completely.
You first have to come to terms with that possibility, however dreadful it might be... I suggest you spend more time (if you aren't already) with your Grandpa so that you are comfortable talking to him and know warning signs of a possible anger outburst.
It might be stressful for your fiancee if you told your Grandpa with her there; though on the other hand it would make the marriage and coming out that much more real and true to him. You both would need to be very humble, smiling, confident but not brash.
It is your choice to bring your fiancee or not for that day. Just make sure your grandpa knows you care about what he thinks- that's my opinion.
Best of luck, my heart goes out to you, and congratulations!
The one who holds on to anger is the one who gets burned- Buddha