In a liberal context, marriage means nothing except for being a symbol of a union between two people. But all rules, obligations and rights that make marriage a meaningful institution are rooted in religion, and are hence not always respected outside of religion.

You could argue that there are laws that only apply to married couples, and that brings meaning to marriage. Firstly, generally speaking, things we associate with marriage culturally are mostly not protected by law, most notably: fidelity. Secondly, to my limited knowledge, the line between a married couple and two people living together is increasingly getting blurred by laws that apply marriage legal obligations even to non-married couples if they have lived together for long enough.