Well to me it seems as if the issue is that she prioritizes being with her family of origin over being with her husband on Christmas. He is willing to compromise and go to her family over Christmas so that they can be together on Christmas, but she is not willing to do the same. She expects that he will either be apart from her on Christmas, or, if he wants to be with her, he will not get to see his parents on Christmas. I think that is pretty selfish actually, and if, as you say, it is a bone of contention between them, it suggests that she is wielding some power in the relationship. There certainly could be more to it, which is none of your business, but I don't think that you have to twist yourselves into mental knots to excuse her exclusion of you.
Creating memories and togetherness at traditional holidays such as Christmas (it would be different holidays for different families, eg for Hindus it might be Diwali, etc.) do play a part in developing family relationships and fostering happiness and even good health. It's understandable she wants that for herself, her parents, her siblings, but then I think you too deserve the same thing. I have been separated from my parents for Christmas for many years because of where we live, and I will say it weighs on them, and it weighs on me.
To be honest, I would not pretend that this does not bother you, especially to your son. Not that you should go out of your way to tell him it bothers you, but also I don't think you should put on a fake, martyr-ish, "it's ok, we LIKE playing cards all by ourselves and we won't have to get as big a turkey, we'll be fine." It's not fine. He's allowed to care about you and to want you to be included in traditional family holidays just as much as his wife wants the same for her family. Also, if you give up on togetherness at Christmas, and they build a new tradition where they are both always with her family for that, then you may end up with your relationship with your son withering away, as many people associate traditional holidays with "nearest and dearest" and the people they feel close to are the ones they have those traditions with.