Well I guess I look at things a little differently....
I think we need to display love, even if they can't...
Personaly I would have wrote a letter about my marriage and then about my kids....(their grand kids). It would say something like...
Dear Mom and Dad
I just want you to know I'm getting married next month to a wonderful girl who loves me unconditionaly, which sadly to say you can not. We plan to have children....which I'm sure, your going to want to see, but since I'm not welcome in your home, it would make no sence to let them come over. Has a parent I will always love my children unconditionaly, no matter what religion they choose for themselfs in their later years .....and since you and Dad don't have that ability....I and my family have no deisre to be around either of you....When you stop serving your hateful god give us a call.
Love always your son.