My family history is similar.
My paternal grandfather was Catholic, married to a protestant. Whereas my maternal grandfather was Protestant, married to a Catholic. My maternal
grandfather couldn't stand the Catholic faith at all, yet he married a Catholic woman, and his daughter married my dad (who is also Catholic).
I was brought up Catholic and still have reverence for the faith, despite being pagan myself.
My opinion is that its ok to disagree with others beliefs, but not to generalise. Accept the individual rather than grouping him/her with others. My
maternal grandfather disliked the Catholic faith, but fell in love with a Catholic woman. He welcomed my dad into his home. He recognised that each
person is an individual and should not be judged by the religion they follow.
Talking is the key. Your girlfriend loves you, despite your Catholicism. You love her despite the fact that she doesn't follow your faith. In the end
it is love that will win out. Despite differences in religious beliefs, you are both Christians, which makes things somewhat easier than if one of you
was Muslim, Jewish or Pagan.
In the end it is your
lives and the lives of your future children that counts. Take advice from your family and friends but don't let them
browbeat you into doing something that you would later regret.
Alternatively, you could both convert to Pixianity
and you wouldn't have any
problems with religious differences