Honestly, I understand your religious feelings, however it is so supremely judgemental to refuse to attend a wedding. Doing so, with that attitude, has nothing to do with Christian love, or with understanding.
Having said that, I also understand that you are angry at having to play the “emotional crutch” and subject your daughter to it all. So for that reason, I do not think you should go. And in fact, subjecting yourself to a ceremony that causes you so much discomfort from a moral standpoint, is not something God expects, I think.
If you go to support your DH, you’ll hate yourself and him for it. It sounds like you are not feeling truly supportive, and so will resent it.
If you do not go, you will be sending a rather rude message to a family you already have strained relations with. Not Christian, imo, and not smart. So you have to do something.
So here is what I would humbly recommend, since you asked

- write your DH’s mom a really nice note (a card, perhaps?) and say you won’t be there, because you need to think of your daughter and the complications of coming. Wish her all the best (which you DO, right???), and leave it at that. Let your DH go alone. He’s a big boy. Your going along just helps him maintain his bad relationships.
In the end, the truth is, that no matter how you DISAPPROVE of this wedding, it really is not your decision to make. It is in fact, none of your business who she chooses to marry. God calls on you to behave in a manner showing Christian love - such as a kind note would. But judging her. . . that is His role, not yours, Sweetie.

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