In what religion do atheist are getting maried?

I suppose that depends on a lot of different aspects such as culture, upbringing, and of course, the religion of the spouse-to-be. A marriage and a wedding are two separate entities, though, and you can have one without the other. There's always a civil ceremony, or simply going up to a justice of the peace and putting your names on paper. You could adopt traditions of a religion you like (for example, I liked the concept of hand fasting). In my own case, my wedding was a performed by a Christian pastor and was Christian in nature. This was done to pacify my family and my husband's family, since they are Christian and he believes in a god. It wasn't my ideal wedding, but it was good one nonetheless.

Many people get married in the local city government. iT is just the same.

You assume that all marriages result from a religious ceremony (or must be linked to a specific religion) and that is simply not true. In places like the United States, there is civil marriage and religious marriage. All marriages in the US have the civil requirement because there must be a marriage license but only some people will choose to have a religious ceremony. I did a bit of reading on this and other countries such as Germany and France require the civil ceremony. Having a religious recognition later is at the choice of the couple but it's only the civil recognition that is necessary.

I'm not sure what the custom is in your country, but many people in America just get married by a judge rather than in a church of any kind. It just formalizes the relationship so that everyone knows it is official. This works for people who are not from any religion. Does that help?

An atheist can get married where ever they give pleasure to. Atheists can get married everywhere. It doesn't matter.

In our country legal marriage and religious marriage are separate. You can do the legal without the religious, but not the other way around.