If I may offer up my 2 cents worth... my husband's family eats a very "English" diet, lots of meat, chicken, sausages, butter, bread, potatoes, desserts - plus my father-in-law does the Atkins diet.
Since we've been together, my husband has been eating more varieties of foods and having fresh salads, grains, veggies and cut back a lot on sausages, meat, etc.
I was very honest with my husband's family from the start and told them that I eat a predominately vegetarian diet (I still eat fish so I am not a full-on veggie - hence my interest in Macrobiotics).
I didn't explain my reasons in a judgemental way or to make them feel bad about their eating choices, just that "I am". We still enjoy meals together when we get together on occasions and I ususally either bring a vegetable dish to share with everyone or eat the vegetables and grains that my mother-in-law makes. I may splurge with a little dessert that she makes but I'll have half the amount or have some fresh fruit.
I think that if you are serious about your special someone, and may even be a part of his family some day, it may be best to be honest from the start, if they are centered people, they should respect you for it.
Perhaps let them know in advance so that the cook in the family is prepared, even offer to bring a Macro dish, and that way it won't turn into an akward situation when you're sitting there in front of a thick steak and a tray of mashed potatoes with not a grain or veggie in sight.
