I suspect, though I am not certain, that your grandparents' view is that gay marriage isn't marriage, that they cannot be seen to countenance or respect it, and that the damage to the family is sad and unfortunate but ultimately not particularly relevant to their decision, which is theologically rather than socially motivated.

See for instance Matthew 10:34-37, which while not specifically about gay marriage in particular does seem to deal with matters where religious and family ties come into conflict:

"34 Do not think that I came to send peace upon earth: I came not to send peace, but the sword. 35 For I came to set a man at variance against his father, and the daughter against her mother, and the daughter in law against her mother in law. 36 And a man’s enemies shall be they of his own household. 37 He that loveth father or mother more than me is not worthy of me: and he that loveth son or daughter more than me is not worthy of me."

I'm not sure what group or denomination your grandparents are from, and there are quite varied perspectives on this, but I suspect they still love their daughter and find the circumstances heartbreaking, yet nevertheless cannot take action that seems to them to endorse or condone sinful behavior, even if it leads to sorrow and conflict within their family.

1. A more concrete title might be more suitable like, "My christian family is struggling to stay together and I am looking for ideas".

2. There may be a middle ground like attending the celebration and not the ceremony.

3. I would attend or do 2 as described. Here is my reasoning.

Contextually according to religions a marriage is not just (a) a relationship between two people, it's also (b) a spiritual relationship, and (c) a covenant with god.

For A, B and C. As far as the religion is concerned A is "possible", B is not, and c is not.

For these reasons, it would be blasphemous to "attend a child's wedding" in the sense that you are asking them to do.

Further consideration. As a parent you are supposed to love a child unconditionally. The agreement you make is to raise them into adults that can take care of themselves. This "daughter" is no longer a child, and as a consequence can make their own choices (like marrying a girlfriend). On top of that, sometimes children can do things that can cause adults to disown them. For two examples: pulling a gun on your parent, getting addicted to drugs and stealing their property.

While I think those two examples are quite serious, you have to think contextually about pulling a gun on the religion and marriage as pulling the trigger as being relevant to this situation.

Strictly speaking, the parent-child relationship can be thought of as the same as any other interpersonal relationship with a few extra strings and caveats. If you had a friend who slapped you across the face every time you met up with them, first thing. As a greeting. you'd probably stop seeing them so often.

If you are finding it hard to understand what they are going through, these analogies might have helped. And yes, they sound ridiculous from your perspective, but from their perspective the daughter is forsaking the religion. It's a death/hell sentence. That's a big deal.

My advice to you is to look at pro-gay christians for guidance to bring that branch into your lives. "God loves all of his children" is a saying that gets passed around. That may come in handy.

If you want them to change their mind, I would make sure to listen to them and be compassionate to the stress they would be going through. I want to emphasise that it's not the daughter's job to do that and external (professional) counselling and mediation could be handy here.

To that end - consider for each party, what outcomes they may want. It may look something like this.

Daughter:
My parents to accept who I am and be happy for me.
My parents to openly attend my wedding.

Parents:
My daughter to not go to hell in the eyes of my religion
My daughter to not embarrass us by getting married and making us attend.

Yourself:
My family to be able to all be in the same room together without tension around.
My family to not fall apart.

Then take these outcomes or in fact the real ones, to mediation. And talk about how to get to those goals.

Good luck (PM me if you would like to chat)

People go through a grieving process when their image of a loved one changes in a way that they perceive as negative or shocking. That process can be very long. It's possible that your grandparents won't be able to get though enough of that process in time to attend their daughter's wedding, or even at all. And if they don't have it together enough to avoid negative emotional outbursts at the event, it may not be for the best if they attend.

If they made this decision in only an hour, however, I think it would definitely be worth encouraging them to sleep on it. The engagement probably is a shock, even if it should be unsurprising; they may have been holding some rationalizations that underplayed the significance of their daughter's relationship.

Even assuming their views on homosexuality never change (they probably assume that, so assume it for the sake of argument), they may eventually regret missing a significant family event. At some point, if they want to have a good relationship with their daughter, they're going to need to make peace with persistent disagreements. If your aunt is considering raising children, maintaining a good relationship with her (and her partner!) is a prerequisite to having a good relationship with those grandchildren. Given that, your grandparents may want to put some work into getting to a place emotionally where they can be happy attending their daughter's wedding.

(Their views on homosexuality may eventually change, too. But trying to persuade them on ideological grounds is more likely to get them to dig in their heels. The most effective persuasion on those grounds is often passive and long-term. Sometimes emphasizing emotions (e.g. people will be sad and disappointed if they don't attend) can be effective, but that may just remind them of their own negative emotions. Focusing on relationship goals is often a good idea when trying to mediate this sort of conflict.)