Well, this continues to be a problem in most parts of the world, including Nigeria. Personally I do not see any problem in allowing my daughter or son to marry the one they love regardless of religious affiliation.
There is no easy solution to this problem. Time is the only sure solution. Presently all you could do is to show to the parents of your girl your commitment/love and why this is more important than religious affiliation.
"Time is the only sure solution." Yes. In in the next 25 years, awareness will make people to allow religion to be strictly personal. Individual's personality/charater will become more important then than religious affiliation.
We just have to wait and see then. For now, all I can say is I wish you good luck.