I think the whole point is to have a light heart about this stuff. When someone is obviously brimming with joy and smiles and wishes me Merry Christmas, I smile back and wish them the same.
When someone gets all cross, and tells me that they are wishing Merry Christmas whether it offends someone or not it leaves a nasty taste in the mouth.
I don't know how I'd feel. Probably somewhat bemused the first time it happened. But if it just kept happening, I wonder how long before my bemusement gave way to something else.