Religion is one aspect of a person's value system. It really isn't possible for a religious person to be with a someone secular or of another religion... no matter how accommodative you may be. Eventually you will get tired of fighting for your right to be yourself, or tired of being something she wants you to be. Sex is only the beginning. Just as you want your children to form their own beliefs... she will want to instill in them all the values of Christianity without any regard to what you may have to offer your children. You may say you're ok with this now, but I have seen marriages end almost immediately over differences of parenting.
You will not be able to convince her to modify her beliefs and neither can you truly give up your own. The differences in this relationship are much too extreme for both to compromise and be happy with the result. You will always feel like you're losing or she will. And this will just breed resentment over time. It would be best to move on and find someone with a similar belief and value structure as yourself.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen