Marriage is both a legal and religious ceremony, but it doesn't have to be religious. Many people, atheist or not, opt to be married simply by signing a legal document in the presence of someone with the license to marry. (The couple must qualify of course)
The only thing that is legally bidding is the marriage certificate from a justice of the peace, the rest is just ceremony and pageantry. Theists are missing out though, they all have to do the same old church thing that everyone else does. I've been to some awesome atheist ceremonies, a Lord of the Rings Elven theme, and an Alice in Wonderland theme. They were a blast, both were outdoors and the ceremony and celebration were at the same place.