I don't know how marriage is handled in the US, but in Germany it is more than just a religious or cultural thing. It has several legal consequences, for example if your spouse is unconscious in the hospital, you are (officially) only allowed to visit if you are family. There are also tax breaks for married couples etc. I think the legal reasons would be a main reason to marry, after some point, the state basically forces you to marry because it is kind of stupid to not be allowed to visit your ill spouse.

Personally I despise the hypocrisy of the state when it forces a certain value system on us with the marriage laws (like tax breaks for married couples - so the state takes a stance against unmarried couples for moral reasons).