Why aren't Christians lobbying to prevent Atheists from getting married?

After all, the Christian concept of 'traditional marriage' is a biblically based one. The marriage, to Christians is supposed to be a reflection of the relationship between Jesus and the Church. If s shouldn't be allowed to marry because the bible says uality is a sin... then why shouldn't Christians be getting all antsy about Atheist marriages since they are secular and do not represent what they believe marriage should be any better than a marriage would?