Isn't marriage just a religious thing? Why would atheists even get married?
There is a common perception that marriage is a fundamentally religious institution that it is based upon religious values and exists to serve religious ends. Thus, if a person is not religious, then it might seem natural for that person to avoid entering into marriage - and that would include many atheists as well.
The problem is, this perception of marriage is rather inaccurate. It is true that religion has a lot to do with marriage as it is commonly practiced in many countries, including the United States, but that doesn't mean that this relationship is inherent or necessary. The key to this question is understanding that the way things are normally done is not necessarily the way they must be done or the way you should do them.
Marriage ceremonies have two related aspects: the public and the private. The public can be regarded as the legal realm where marriage is sanctioned by the government and where married couples receive certain economic and social benefits. The private realm involves the creation of a new family unit: when two people marry, whether that marriage is official or purely personal, it is a serious expression of love, support, and commitment between two intimate individuals.
Both the public and the private aspects of marriage have their importance; neither, however, requires a religious basis or even religious involvement. Although there are many people in society who will try to act as though religion and, in particular, their religion is an indispensable factor in both the public and private realms of religion, you shouldn't believe them.
With the private realm, some will argue that reliance on God and adherence to various religious doctrines are key ingredients for creating a successful and happy marriage. Perhaps for members of those religions this is true if one is a devout believer, then it seems unlikely that they can participate in such an intimate and important relationship as marriage without their religious beliefs coming into play.
However, that does not mean that two people cannot build a solid, long-lasting, and very happy marital relationship without religion or theism playing any sort of role at all. Neither religion nor theism are necessary in order to be intimate with another person. Neither are necessary in order to love another person. Neither are necessary to be committed and honest with another person. Neither are necessary to create a sound economic basis for a relationship. All in all, neither religion nor theism add anything to a marriage unless those involved already rely upon them in some fashion.
With the public realm, some will argue that particular religious conceptions of marriage are and have always been necessary for a stable social order; as a result, only those conceptions of marriage should be officially recognized by the state. Because of this, not all committed relationship receive the economic and social benefits of marriage.
The fact of the matter is, however, the current Western notion of marriage as being only between a single male and a single female is culturally and historically conditioned - there is nothing very necessary or obvious about it. Other types of marriage can be just as stable, just as productive, and just as loving. There is no reason to eliminate them from the category "marriage" except, perhaps, as a means to promote religious or cultural bigotry.
None of this means, of course, that two people in a committed and loving relationship must get married. There are important advantages to having the marriage certificate and there seems little reason not to do it if you are able, but if you continue to have philosophical or political objections then that is perfectly fine. Not being married is no more a barrier to having a deep and meaningful relationship than is not having religion.