How does a difference in religious affiliation make someone undateable?

Devout is the keyword. People who are truly devout in religion, insist that their families are also. One of the many reasons is the afterlife reasoning. I'm sure this girl is thinking that if she marries you, you could influence the children you have together into becoming atheist and that would prevent them from going to heaven. Remember, most women date you to evaluate if you are husband material.