I completely disagree with that approach about “seeing grandkids.” While I know later in the thread you have stated that this is not a form of revenge, refusing to allow people to see their grandkids is a highly emotional and illogical response to an action that is not necessarily “making a big show.” I suppose it depends on the way the non-attendee approaches not attending. If they were to say, “I’m boycotting your wedding, I don’t respect that this couple is a couple, and I am going to make a stink about it to people” then yes, you would have a major respect problem. But if a parent said, “I love you as people and as a couple, but I don’t believe this wedding ceremony will create a valid marriage and therefore I can’t in good conscience attend, smile, clap, cheer, and accept congratulations for something that I believe is not actually occurring. So while I respect your choice to do it this way, I have to stay home that day,” then I believe you would have an obligation to respect their decision, even if it is frustrating. Aren’t we do respect each other’s freedom to follow their conscience?
But what does that have to do with starting a family? I think premarital sex is wrong, but if an unmarried woman has a baby, I acknowledge that she has started a family. Therefore, if an invalidly married couple has children, I would also acknowledge that they have started a family. One doesn’t need the sacrament of marriage to have a family, one needs the sacrament of marriage to be married. The two are not exactly the same thing.
So let’s move on to the day when your child asks you why she doesn’t ever see Grandma. It is a pretty weak answer to say, “well, Grandma didn’t agree with us getting married on the beach and didn’t attend the wedding, so we won’t ever let you see her.” If I were the grandchild, I’d be pretty upset that one disagreement about a wedding would preclude any relationship at all.
Now if Grandma said that she would take any chance she got to teach the grandchildren that the parents were wrong? Or if Grandma didn’t respect you as a person? Or was undermining your relationship with your husband? Then I would understand that need. But if that grandparent is NOT setting out to harm the grandchild (in the form of insulting or undermining or indoctrination etc.), but just couldn’t attend a particular event, then why would there be a need to keep them separate? There isn’t, other than punishment.
Wanting to spend less time with them? Reasonable.
Choosing not to spend tons of money on plane tickets? Understandable.
Actively preventing them from ever seeing their grandchildren? Emotional/Punishment. Actively preventing grandchildren from a connection with the harmless older generation? Big bummer for them and potential for backfire when the kids get old enough to resent it.