Marriage does not require religion anymore. As a matter of fact, a marriage in church does in most countries not even have a legal status any more. Most churches or similar institutes do require you marry for the law first before you can marry in church.
That being said, atheists can just get married like everybody else, since the church and all religious rituals are all optional these days and have no legal status, only the law matters. And an atheist is not necessarily an anarchist. 😜