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 THAT brings meaning to marriage. But:

Firstly, generally speaking, even the most important features of a marriage are not protected by law, most notably: fidelity. So the law is disjoint from what's traditionally considered to be obligations within marriage. That leaves the legal definition at the whims of contemporary polititians. Therefore, law cannot assign the word "marriage" any consistent meaning throughout time.

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. It suggests that law-makers do not consider a ceremony and a "marriage" announcement to be what should really activate these laws, but rather other factors. Although, they seem to acknowledge that an announcement of a marriage implies the factors needed to activate these laws. If that makes sense...

So marriage is inherently a religious institution that in a religious context comes with rules, obligations and rights. Hence why people who take religion seriously will find it offensive that somebody that completely disregards these rules calls themselves married.