I'm declaring meself with the Atheist Front too, and see no problem with following a few quaint traditions at Christmas that, for me, are just that - quaint traditions. No problem in loving Christmas because it's the family / friends / feel-good vibe that I love, and most of the Christmas traditions that we observe, from mince pies to Coca-Cola coloured Santas, are secular in origin anyway.
As the g/f is moderately religious I forsee partaking in several Christian ceremonies in the future which I have no problem with as it'll be for her. I'll hardly have her convinced in that short a space of time. :D
To be honest, there's many more hypocrisies involving the church you could be getting worked up about. I had typed a few out, but that argument is for another time.
Oh, and we're not all Celt by the way, surprisingly few of us are. There's a similar level of Viking blood here, for example, and rather a lot more Anglo-Saxon.