Politely refusing is out of line? It's so hard for Christians to understand that not everyone is a Christian. This comment is a good example of why religion should stay off the softball fields.
I like the old saying that you should never discuss sex, politics or religion in polite company. I think that should apply to the softball field as well.
My one exception to the religion part, if it is a church league, I would expect there to be a team prayer. That is the right time and place for it.