hmmm...sounds a bit more nihilistic than athietist to me. dash of one and splash of the other i guess.
as a devout athiest, i can't provide you a single moral or spiritual reason to get married. you can make a commitment with out signing a piece of paper. if you want to sign a piece of paper buy a house together. there's a lot more paper to sign and its just as complicated to get out of that contract as it is to get a divorce.
that said, i'm happily married. we did it for our own reasons. none of which have to do with traditional family values. we did spend a very long time dating and living in sin. it was good. it still is. your relationship won't get better or worse b/c you get married or you don't.