For firstborn sons who are now married...?

You don't mention how old you are, or exactly what your mom's objections are to your girlfriend. If they have merit, then take that into account. Otherwise, if you're an independent adult, then tell your mom she'll always be welcome at your Thanksgiving table and live your life as you see fit; either she'll come around, or she'll just have to miss out on account of her own stubbornness. If you're not, then hold off on marriage or any discussion of it until you are ready to make that declaration. But either way, you can't change her attitude. All you can do is stand up for what you know to be right and your ability to make your own decisions, and hope that she eventually comes to see things your way. If she doesn't...that's unfortunate, but again, it's her loss.